MOHAWK: Stopping the handling of mouseclicks if the cursor isn't visiible.

I fixed this bug:
https://sourceforge.net/tracker/?func=detail&aid=3488327&group_id=37116&atid=418820
The problem was that you could click on items on the screen all the
time, even if the cursor wasn't visible. That caused multiple animations
run simultaneously.
Now I fixed it: the engine handles the EVENT_LBUTTONDOWN event only if
the cursor is visible on the screen.