Extract some XAML icons from the VS2017 Image Library. There's no
particular reason to prefer XAML over PNG, but somehow it feels
more forward-compatible. (OTOH, defining images as ControlTemplate
instances is just weird. Yay WPF.)
Restoring the selection works pretty much like it used to, though
I'm capping the total size of the selection because it goes into
stupid-slow mode if you have too many elements.
Getting the item that is at the top of the ListView is astoundingly
obscure, due to the ListView's extreme generality. I make a
simplifying assumption -- that we're using a VSP -- which allows us
to use a simple vertical offset once we've dug the appropriate
object out of the visual tree. (The alternative is to walk through
the list of items and see what's on screen, or perform a hit test
calculation in the upper left corner.) Yay WPF.
