Said function in genericDisplay.js was returning the index of the
actor based upon its position in the entire list, while everywhere
else indexes relative to the currently displayed page were used.
This made actions in the details pane break (bug #590949), so I
replace it with a new function in shell-overflow-list.c,
shell_overflow_list_get_actor_index, which is page based.
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).
