MouseCrossed is called when the mouse either enters or leaves the Area. It is called even if the mouse buttons are being held (see MouseEvent above). If the mouse has entered the Area, left is false; if it has left the Area, left is true.
If, when the Area is first shown, the mouse is already inside the Area, MouseCrossed will be called with left=false. TODO what about future shows?
MouseCrossed(a *Area, left bool)