This patch adds a magenta rectangle around the currently inspected
widget. This allows you to browse an app's widget tree somewhat
visually using the Inspector. :^)
This allows windows/widgets to learn when something is being dragged
over them. They can then repaint themselves somehow to indicate that
they are willing to accept a drop.
Currently this is piggybacking somewhat on the mouse event mechanism
in WindowServer. I'm not sure that's the best design but it seemed
easier to do it this way right now.
