Instead of LibGUI and WindowServer building their own copies of the drawing
and graphics code, let's it in a separate LibDraw library.
This avoids building the code twice, and will encourage better separation
of concerns. :^)
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.
