The various items in the Actions menu are enabled or disabled based
on the current selection. There's no SelectedIndices property in
WPF ListViews, so we have to do things slightly differently.
The SelectedItems list isn't kept sorted to match the list contents,
so finding first/last item requires a bit of scanning.
Also, rearranged some stuff. I'm trying to keep the old and new
code somewhat parallel, to make it easier to walk through at the end
and see if I've missed something.
Renamed VirtualListViewSelection to DisplayListSelection, because
it's now tied to the DisplayList implementation. Hooked it up to
handle SelectionChanged events.
Also, tweaked the code list item style to remove the one-pixel gap
between items. Somehow I manage to click on the dead zone with
surprising regularity.
