WIP: Plugins: Add xfconf plugin

Related to #60 (closed), xfce/xfce4-settings#256 (closed).

Currently this simply makes the dependency to xfconf optional, using a binding to the monospace-font-name GNOME setting instead (to which is bound xfconf since Xfce 4.16). I'll see what integrating Mousepad settings with xfconf can do next.

My idea is that the default Xfce text editor should be next to the file manager and terminal when the user opens xfce4-settings-manager, and that he should have access to the advanced Mousepad settings when he opens xfce4-settings-editor.

This could also allow to include Mousepad in an export/import of Xfce settings based on xfconf, as mentioned in xfce/xfce4-settings#256 (comment 25307).