Use windowNumberAtPoint: to determine if a mouse event is on a window.

If we've presented a modal window from a popover, we don't want it to be dismissed when the modal window is moved ... the mouseUp event from moving the modal will cause the popover to be closed. :cry: 

Not yet convinced that this is the best solution, so I will try out a few other ideas.

**UPDATE:** Now using `windowNumberAtPoint:` which can tell us which window a mouse event will hit, instead of just checking if the mouse event is within the window's frame.