Change window highlight style in WindowHeapDelegate

Currently FrameSvgItem is used to highlight selected or hovered windows.
But the problem with it is that it's a bit heavyweight. As a way around
it, we could put it in a Loader, but that's going to be bring a set of
other challenges.

As an alternative solution, this change replaces FrameSvgItem with a
simple outline. It still produces decent visuals and it's simpler.