gdk: Move window beeps into GdkWindowImpl

gdk: Move window beeps into GdkWindowImpl

One less magic function. Also refactored it to make it easier to
implement. It now returns TRUE if it beeped and FALSE if it failed to do
so. A default implementation exists that just returns FALSE for all the
backends that can't beep windows (read: everything but X11 with XKB -
and why on earth do keyboard libs implement beeping?)