applets/kickoff: change category at most once per frame
a possible cause of the freeze that some people experience when switch on hover is enabled and moving the mouse very quickly over the categories might be that many category changes happen per frame if the mouse polling is much more than the framerate. this introduce a slight delay to not change categories more than ~60 fps, perhaps could be made even a bit slower or we could actually hook to the frame changed signal
Edited by Marco Martin