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. :^)
WSWindowManager::invalidate() had a bug where it would mark the entire screen
rect as dirty, but it wouldn't scheduled a deferred recompose.
This would cause any subsequent calls to invalidate(Rect) to be coalesced
with the pending compose, but the pending compose never happened.
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.
