Don't include NoDisplay items, except for window identification

Don't include NoDisplay items, except for window identification

Searching across NoDisplay desktop items can produce weird
results to the user (including duplicates, and items that
aren't really applications at all.) So, don't include them
normally.

But continue including NoDisplay items when we look up the
desktop file for a window, since we want to catch applications
like Evince and Nautilus which are otherwise NoDisplay.

http://bugzilla.gnome.org/show_bug.cgi?id=587548