Merge in GTK3 port. Special thanks to Simon for appearance fixes, widget cleanup, and suggestion of the WindowScalingFactor setting. Summary of changes:
- All components and glade files have been ported to GTK 3.20, all deprecated widgets and functions replaced
- Build requirements have been bumped to gtk+-3.0 3.20, glib-2.0 2.45, gio-2.0 2.45, gio-unix-2.0 2.45, exo-2 0.11.0, libxfce4ui-2 4.13.0, libxfce4kbd-private-3 4.12.0
- Added support for /Gdk/WindowScalingFactor

Squashed commit of the following:

commit 9d52b25530c7f88d86573c7e6cf0c8e938702107
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 22:34:10 2016 -0400

    Upgrade all glade files to gtk 3.20, replace deprecated widgets

commit 9ccbace18fc19f03c9cfd5959f8f48d6cb865ad0
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 20:46:27 2016 -0400

    Bump required versions to support GTK 3.20 and Xfce 4.13+

commit ca182a87113bf0e2720420ce99fc58cfe2dbe223
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 20:18:11 2016 -0400

    Fix drawing issues (again)

commit eb5b63cb505b5fd311a075410607c13a2b72645c
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 18:11:25 2016 -0400

    Current progress, runs, no display

commit 3d76c9fe59e70a207bf9ee76a35e9d48276494d6
Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Date:   Wed Sep 14 23:31:35 2016 +0200

    Drop unused vars

commit fdf7cc3da5d889e590f76b6e3cb9b31f49e29056
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 06:27:11 2016 -0400

    Fix deprecations

commit 59b3ea798178240f13168cf4e10d56ff894a965b
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 06:11:53 2016 -0400

    Fix paint callback

commit 07753d65d89193149a0bf609ef50c0b6e405fca4
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 06:02:41 2016 -0400

    Replace expose event with draw

commit 559abb683b2dd6ad5df7b6bc398941aba12f7dd2
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Wed Sep 14 05:56:00 2016 -0400

    Discard IFDEFs

commit 8d1d5af66c9df099c952015fa050f90c8488b169
Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Date:   Wed Sep 14 11:20:27 2016 +0200

    Fix background color of scrollarea

commit c1048c7a0eea7d58b511d3ca4ad37970fd04a416
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Tue Sep 13 22:52:21 2016 -0400

    Replace string

commit 6fd34046d40315839b65bbb82e6a79b90cb49630
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Tue Sep 13 22:48:17 2016 -0400

    Support Gdk WindowScalingFactor

commit 9df1063fee837128fe78070900d19544a20b6f17
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Tue Sep 13 22:03:10 2016 -0400

    Import latest foo-marshall and scrollarea from mate, builds with --disable-debug

commit 2bb20c12fcd36e3e567a28f338423556f730316a
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sat Sep 3 06:13:55 2016 -0400

    Drop definition of xfce_widget_reparent, bump required libxfce4ui version to 4.13.0 which now provides it

commit 03b2bf87e7702f9c89760417d27c8c86a199b1c7
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sun Aug 21 21:35:14 2016 -0400

    Partially port display settings

commit 1316fb377b3fa2c5b42060981385c1909c634a16
Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Date:   Mon Aug 8 00:22:33 2016 +0200

    Drop all deprecated widgets from the a11y settings dialog

commit 8b36f4ce5c834eed09b7e18ed7235e58e4311e64
Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Date:   Mon Aug 8 00:02:22 2016 +0200

    Drop all deprecated widgets from the mouse settings dialog

commit c5d40bf125b0a05ec4c2fad2c03bfdbf78ac3443
Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Date:   Sun Aug 7 23:48:27 2016 +0200

    Drop all deprecated widgets from the keyboard settings dialog

commit f017811a1e25722e8e7233a2f323c0e342973710
Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Date:   Sun Aug 7 23:40:50 2016 +0200

    Drop all deprecated widgets from the appearance settings dialog

commit a90e9c10848dc81da234b424b121b6b4af30ef8d
Author: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>
Date:   Sun Aug 7 23:15:09 2016 +0200

    Fix background styles of header and viewport in manager

commit 02097c9c1052f4639e79488113b73eeb496bd7c7
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sat Jun 18 14:17:29 2016 -0400

    Use text renderer to fix display issue

commit 9fff5000548ed99a4bf65f9862e50bd201552c0f
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sat Jun 18 12:54:36 2016 -0400

    Initial broken port of the settings manager

commit 7f98f7d11624dac51dcc2e67e2d744ba36b40b3f
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sun Jun 5 18:42:10 2016 -0400

    Mostly fix colors, selected color is still broken for some themes

commit 298974550d7ee2164fe0d1b5db3fec8b4b05de5b
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sun Jun 5 14:01:42 2016 -0400

    Port appearance settings. FIXME: showcase colors are always black

commit 3487e364d7a2340924ffe25dcda212cdcf0ae1e5
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sun Jun 5 08:25:35 2016 -0400

    Port xfce4-settings-editor

commit 95f86ffe126b8110d0fcf6c4a349a3b6e4bd2d61
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Tue May 31 21:50:47 2016 -0400

    Port xfsettingsd to gtk3

commit 8beea08b96fbfad32c9e6aa8fbc0068e656c6164
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Tue May 31 21:09:39 2016 -0400

    Port xfce4-mouse-settings to gtk3

commit c2db3c3a1c448339424077ab1a2168643c77c385
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Tue May 31 20:48:38 2016 -0400

    Port xfce4-keyboard-settings to gtk3

commit 3648eece22e403daf4710f24f4b3f00437a4adc9
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sat May 21 09:44:56 2016 -0400

    Drop unused libxfce4util references in the keyboard settings

commit 11d60fde191cfd0b8704dd7baa181b8ea8273747
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sat May 21 09:18:26 2016 -0400

    Port xfce4-accessibility-settings (plug functionality untested)

commit 4e9fac5214f30584e770e02b04fa22c5f59bbc3a
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Sat May 21 08:17:13 2016 -0400

    Complete port of xfce4-mime-settings

commit 859d8a62fa330bc9f3c95c2d0964352bfa346541
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Thu May 19 22:19:50 2016 -0400

    Partially ported xfce4-mime-settings, does not build

commit 4ac0151520eed29b2d3bf5f8bffe4602b1bc0d66
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Thu May 19 21:18:39 2016 -0400

    Further seal and replace deprecations, should now be ready to port

commit ba64a334c90a1caf5562233d22df67bdc1bfca24
Author: Sean Davis <smd.seandavis@gmail.com>
Date:   Thu May 19 20:39:13 2016 -0400

    Build with GTK_DISABLE_SINGLE_INCLUDES, GDK_DISABLE_DEPRECATED, GTK_DISABLE_DEPRECATED, GSEAL_ENABLE where possible