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.
Allow setting the scope for a controller. The scope determines at what
point in event propagation the shortcuts will be activated.
Local scope is the usual activation, global scope means that the root
widget activates the shortcuts - ie they are activated at the very
start of event propagation (for global capture events) or the very end
(for global bubble events).
Managed scope so far is unimplemented.
This is supposed to be used to replace accelerators and mnemonics.
