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. :^)
I'm going with a global top-of-the-screen menu instead of per-window menus.
The basic idea is that menus will live in the WindowServer and clients can
create menus via WindowServer requests.
