Previously the focused widget would only get cleared on replacement or
on destruction (being a WeakPtr and all.) This could lead to window
dispatching events to a focused widget after it had been removed from
the window's widget tree.
The same issue existed for the hovered widget, etc. So this patch
makes sure that we eagerly clear the various widget pointers in Window
immediately when they are removed from the window's widget tree.
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.
