Possible to make startPlayingFile async or cancelable?

Corrupted MP3 files can cause the startPlayingFile function to block for quite a long time (>10s, but that's probably a conservative value). Since we work with SD-cards, this is not uncommon to happen. This behaviour can really mess up the useablitiy of the player.

It would be nice to be able to cancel playback if it hasn't started after n secods (say two seconds perhaps), so we can skip to the next track.

Might be related to
- #25

Start at the startPlayingFile function in the library and review the related issue #25 for context. Determine how playback startup could be interrupted after a timeout so a caller can skip a track, and verify that normal playback remains usable while stalled or corrupted files can be abandoned.

Written by the indexing model from the issue text.