Better make a separate commit for that too, as I'm still removing quite
a lot, beyond just sorting.

Fixes: 8d2b1eae

This can happen if the panel is hidden at startup because it is
associated with a disconnected monitor, then shown when this monitor is
connected. The plugin's size negotiation then takes place before calling
construct(), where the screen is initialized.

It's probably better in general, but especially since this 7924c969:
since the #ifdef is no longer based on GDK_WINDOWING_*, it can cause
macro redefinitions which, if done the wrong way round, could activate
code that shouldn't be.

* Use XDT_CHECK_OPTIONAL_FEATURE
* Only GTK Layer Shell remains as a truly optional package, and
  therefore HAVE_GTK_LAYER_SHELL as a compiler switch, in addition to
  ENABLE_X11 and ENABLE_WAYLAND.

This doesn't make sense. Libwnck handles this internally without any
obvious way for us to get the number of rows, so it's best not to allow
the user to change the number of rows in this case, as this will only
lead to bad results.

Completes: eeb9a578

It's simpler and getting the scale factor from the GdkWindow does not
work when the widget is not realized.

Closes: #789

It seems that the number of workspaces contained in a viewport is the
ratio of widths, at least when there is only one line. Not sure we
really support other cases.

Closes: #789
Fixes: 8f310b86

This no longer seems necessary after the switch to libxfce4windowing for
some reason, but it could become necessary again, and it's not a problem
to give libwnck a little time to update the layout.

Fixes: #784

If there is an issue with XfceTitledDialog and GtkBuilder, the latter
will report it by its own means, perhaps by returning NULL, which is
then checked.

In the current state of the code on the master branch, this is probably
not necessary, as Libxfce4windowing >= 4.19.1 no longer uses wnck icons.
But it does reduce the distance from the 4.18 branch, where this change
is made necessary by the use of `wnck_set_default_icon_size()` in
tasklist and windowmenu.

Also, if plugins are forced to run as internal via "force-all-internal",
it is possible that one of them will change this icon size. And in any
case, it is probably better to use this API when possible, since it
allows to fix a deprecation.

Related: #695

Except for WnckPager, which will not be ported to Wayland in the near
future.

Using WnckHandle would imply changes impacting the code structure (at
least for the panel), and it is not obvious that this will ever be
necessary at this stage.

This preserves the possibility offered by Libwnck to have one layout per
WnckScreen.

Completes: bb2aaf09

For internal Libwnck reasons, the workspace layout must be common to the
different instances of the plugin. So the best we can do is to
synchronize them properly, as much as possible. The number of rows
synchronizes well, but the orientation does not, which is why it is left
as it is in this commit.

Closes: #7
Related: !82, f1c08ac7
See-also: https://bugzilla.gnome.org/show_bug.cgi?id=635341

Fixes critical warning when there is a screen layout change after the
plugin has been removed.

MR: !82

Fixes: #257
MR: !82

MR: !82

Fixes: #253
MR: !82

Fixes: #209, #274
MR: !82

This reverts commit 57c1c5f7. #253 will
be fixed again in next commits.

MR: !82

This reverts commit d5c3746a.

`wnck_pager_set_scroll_mode()` is available since 3.36.0 and 3.40.0 does
not exist (next release is 40.0).

Closes #253