Previously it would just close the window on MouseDown. Now we do the normal
thing where we require a MouseUp inside the button rect before committing.
This was a bit painful to get right. The code is a lot more pleasant to
deal with now that all coordinates are relative to their local system
instead of being absolute screen coordinates.
The window frame is an object that contains a window, its title bar and
window border. This way WSWindowManager doesn't have to know about all the
different types of window borders, titlebar rects, etc.
