Add a GtkSetting for whether the desktop shell is showing the desktop
folder icons.
This is on by default because most desktop shells do show the icons on
the desktop. We already have a patch in gnome-settings-daemon to bind
this to the org.gnome.desktop.background show-desktop-icons GSettings
key which is off by default on GNOME.
https://bugzilla.gnome.org/show_bug.cgi?id=712302
Add the concept of shell capabilities, which allow the compositor
to advertise support for the app menu and the global menubar,
which are then propagated as GdkSettings.
https://bugzilla.gnome.org/show_bug.cgi?id=707129