shortcutcontroller: Add GtkShortcutScope

shortcutcontroller: Add GtkShortcutScope

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.