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.
My needs are really quite simple, so I'm just going to add what I need
as I go along. The first thing I needed was a simple box layout with
widgets being able to say whether they prefer fixed or fill for both
their vertical and horizontal sizes.
I also made a simple GStatusBar so FileManager can show how many bytes
worth of files are in the current directory.
