This is mainly for internal use, but I can't see a reason to not have it
public for people who want to maintain their own lists.
I'm sure gnome-builder will never ever find a way to misuse it.
Mnemonics need to be triggered with help from the controllers (who
determine the modifiers). Support for that has been added, too.
Mnemonics do not use this yet though.
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.
This is a very barebones controller that currently does nothing but
activate the binding signals. Yay.
And because we have bindings on every widget (Yes, a GtkGrid has a
keybinding - 2 in fact), we need that controller everywhere.
