Each GAbstractView now has a GModelSelection backed by a simple
HashTable<GModelIndex>. When the selection changes somehow, the view
gets notified via the notify_selection_changed() callback.
In the future it will probably make sense to move to using some kind of
ranges as the internal representation instead.
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. :^)
