console: add unregister_displaychangelistener

Also change the way the gui_timer is initialized: each time a
displaychangelistener is registered or unregistered we'll check
whether we need a timer (due to dpy_refresh callback being present)
and if so setup a timer, otherwise zap it.  This way the gui timer
works correctly with displaychangelisteners coming and going.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
