Filter search results

[zwometer]

Implement functionality to filter search results:

• deselect certain sources. (e.g. deselect Spotify to exclude Spotify results)
• select certain sources (e.g. only select Spotify to only show Spotify results)
• change filter AFTER completing a search (1. enter search term, 2. hit enter, 3. wait for search results, 4. change filter to refine the results)
• filter for metadata like length, release date, artist, ... (if possible)

[mariolukas]

done #70 fead1fb