trivial: Sync the spec file with Fedora
Do not show local applications that do not have an icon specified
Warn when overwriting metadata keys that already exist

Also, ignore setting an existing key with the same value as we're trying to set,
we don't want the memory address to be changed for no reason.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=723702
Use a per-list sort key when randomizing a list

If we use the same sort key, applications in multiple lists (e.g. popular and
installed) will be sorted incorrectly. Also, as we're now adding more metadata
that's only going to be used just once, remove the sort metadata after the list
has been ordered.