Add basic filtering on search results

Any MPD query was treated the same, whether it was an album, artist, track, etc. query. With this commit, a basic filtering occurs, meaning that 'album' queries will only return albums, 'track' queries will only return tracks, 'artist' queries will only return artists, and 'any' queries will return all of the above.

Limitation: any other query now won't return anything. This seems logical, since Funkwhale search only distinguishes between artists, albums and tracks.