Selection support for the VirtualTreeControl. The motivation for implementing this here, rather than allowing the underlying listbox to handle it, is that we need to fire the correct WinEvents for selection/focus. If we let the listbox handle it, it will fire WinEvents with it's own child ids that assume a flat list structure. We need to be able to account for hierarhcy in the tree. Also, it gets us a step closer to removing the listbox dependency altogether.