Meta::Track, playlist: show reason why a track is not playable

1. Add notPlayableReason() to Meta::Track (and provide convenience
   implementation of isPlayable() using it)
2. Display track tooltip with the notPlayableReason() if track is not
   playable

BUGFIXES:
 * Reason why a particular track is not playable is now shown in
   playlist tooltip; patch by Anmol Ahuja.

BUG: 313649
FIXED-IN: 2.8
REVIEW: 109817