For a managed GtkShortcutController, changing its propagation phase
away from `GTK_PHASE_BUBBLE` currently results in a lingering reference
due to a bug in GTK (gtk#6246). This means the Escape key shortcut
remains active even after leaving the Display panel, resulting in a
crash when pressing Escape in any panel after leaving the Display panel.
This is now fixed in GTK (gtk!7115), but there seems to be no reason for
changing the propagation phase at all anyways. Since the callback
returns `GDK_EVENT_PROPAGATE` when it does not activate, the event
should still make it to other handlers.
So, to speed up the fix for gnome-control-center, we remove the
propagation phase changes althogether. There is also no need to let the
shortcut have global scope, managed is enough.
Closes#2768
e7e9753aac