This adds an interface for taking care of shortcut controllers with
managed scope.
Only GtkWindow currently implements this interface, so we need to ensure
that we check if any top-level widget we reach is a shortcuts manager
before we call into it.
Reduce the amount of special casing by using a list model
for global and managed shortcuts, too.
This way, the ListModel API will work for the ShortcutController in the
GtkShortcutManager and GtkRoot.
The only special case remaining is shortcut activation, which needs to
pass the right widget to the controller in the global/managed case.
This adds an interface for taking care of shortcut controllers with
managed scope.
Only GtkWindow currently implements this interface, so we need to ensure
that we check if any top-level widget we reach is a shortcuts manager
before we call into it.
