shortcuts: Mananage managed shortcuts with a custom model

shortcuts: Mananage managed shortcuts with a custom model

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.