This patch adds a GShortcut class. Each GAction can have a GShortcut which
will cause the event loop to listen for that key combination app-globally
and activate the event in case it's pressed.
The shortcut will also be displayed when the action is added to a menu.
Use this to hook up Alt+Up with the "open parent directory" action in the
FileManager app. :^)
GMenu now has an "on_item_activation" callback that fires whenever one
of its items are activated. The menu item identifier is used to distinguish
between items.
Use this to implement font switching in Terminal. :^)
