Fixes for GenericDisplay

Fixes for GenericDisplay

GenericDisplay wasn't quite completely converted to the ShellOverflowList
model.  Since the list now holds all actors, the indexing/wrapping
was incorrect.

Add a property which lets us keep track of how many items are displayed,
use this in genericDisplay.

Avoid setting selectedIndex to -2 when going up with no items.

If we're not displaying any results at all, don't attempt keynav (for now).