Any GWidget can have a tooltip and it will automatically pop up below the
center of the widget when hovered. GActions added to GToolBars will use
the action text() as their tooltip automagically. :^)
I'm gonna want to have nested event loops sooner or later, so let's not
pollute GEventLoop with things that are meant to work globally.
This patch also changes key events to pass around their modifiers as a
bitfield all the way around the system instead of breaking them up.
These functions don't exit immediately, but rather on the next iteration
of the event loop.
Since exit() is already used by the standard library, let's call it quit()
instead. That way, saying exit() means the same thing here as anywhere else.
