Don't show unplayble tracks in search results

[samcreate]

example: [unplayable] Earth Angel

spotify:track:3iQaSx6G50ptbEnzgsaQQy

[jodal]

The [unplayable] label is really just a hack in Mopidy-Spotify. IMHO, we should add metadata like this to the Track model instead of adding logic to clients based on substrings in the title.