Do not filter the search ListBox twice

We were doing it once in the backends and once in the frontends, and if they
didn't agree you were left with a blank screen rather than the 'No apps found' screen.
Ensure the same app from two different plugins does not show more than once
Parse the keywords in the AppStream metadata
Search the appstream data for name, summary, description and keywords

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=707841