lib: Add a quirk for parental filtering and filter results with it

In readiness for an upcoming feature, add a GsApp quirk to indicate ‘this app has been filtered by parental controls’. Apps will typically be filtered due to their OARS ratings, but in order to keep the policy flexible (and implementable by different plugins), we do the filtering on the presence of a quirk.

Completely hide apps which have this quirk, unless they have been installed already (which we take to mean that the parent has deemed them acceptable for the system, despite their OARS rating).

Policy changes to set the new quirk will follow eventually.

Signed-off-by: Philip Withnall withnall@endlessm.com