x11: Don't crash if a window is on an unmanaged screen

_gdk_x11_display_screen_for_xrootwin() can return NULL. If so, return
NULL from gdk_x11_window_foreign_new_for_display().

https://bugzilla.gnome.org/show_bug.cgi?id=709716
bindings: Fix stray semicolons in CSS example for bindings

Semicolon shouldn't be at the end of binding set
Change the default for "show-desktop" back to TRUE

Change the GtkSettings default for "shell-shows-desktop" back to TRUE
and also change the default value of the "show-desktop" property on
GtkPlacesSidebar so that the defaultvalue test passes.

https://bugzilla.gnome.org/show_bug.cgi?id=712302
defaultvalue test: ignore show-desktop

Ignore the "show-desktop" property on GtkPlacesSidebar for the
defaultvalue test.

Currently, "make check" is passing because it runs the test under a xvfb
with no XSETTINGS provider, so we see the Gtk default value.  No matter
what we set the default value to in Gtk, however, there will be some
desktop environment in which someone running the installed test outside
of an xvfb will get the wrong result.  Best to ignore it.

https://bugzilla.gnome.org/show_bug.cgi?id=712302