Add an XSetting for the session bus ID

This will allow us to identify situations where we are not on
the same session bus as the xsettings provider.

https://bugzilla.gnome.org/show_bug.cgi?id=671802
GtkApplication: Try to cope with ssh situations better

Override the gtk-shell-shows-app-menu and gtk-shell-shows-menubar
settings to FALSE, if we can detect that we are not on the same
session bus as the xsettings provider that we got these settings
from.

We determine this by comparing the bus ID of 'our' session
bus with the one reported in the Gtk/SessionBusId xsetting.
If they are different, then it very likely that we are in an ssh
situation where we see the forwarded X display, but not the
session bus. The D-Bus based menu exporting will not work
in this situation.

https://bugzilla.gnome.org/show_bug.cgi?id=671802
Don't initialize GL if it has been disabled

We've observed hangs of mutter when it initializes GTK+, which
are caused by initializing GL, which in turn makes xwayland
call back into mutter. With this change, mutter should just
disable GL support in GDK, and things will work.
widget-factory: Don't pulse dead widgets

Causes crashes at cleanup and we don't want those.

Also, code gets simpler.
widget: Handle setting clip differently

(1) Get rid of supports_clip flag. All widgets (implicitly) support
clip.

(2) Don't reset the clip to { 0, 0, 0, 0 } before the "size-allocate"
signal.

(3) Make gtk_widget_set_allocation() set the clip (to the allocation).
This ensures that eveyr widget has a clip set.
Note: It overrides previous calls to gtk_widget_set_clip(), while in
3.14 this didn't happen.

(4) As the clip is set by gtk_widget_set_allocation() now, don't set
it after the "size-allocate" signal anymore.

This fixes calls to gtk_widget_queue_draw() from inside the
size_allocate vfunc.
csstransform: Fix comparisons crashing

Transforms of different type are not equal.