Action for musicfolder item type 'track' follows setting for song select action

Solves issue #52
Call setDefaultValues to ensure preferences are initialised correctly.
Resolve a TODO, DateUtils.formatElapsedTime is not suitable.

DateUtils.formatElapsedTime() adds a leading zero if the number of minutes
is < 10 (i.e., "01:00", instead of "1:00").  Remove the comment that
suggests it could be used, and add documentation that makes this clear.

Since makeTimeString() is very similar to formatElapsedTime it seemed
sensible to give them the same name.
Add a progress indicator to the mini-player.

- Increase the height of the player by 2dp to make room for a small
  ProgressBar.

- Use a custom drawable as the Android drawables for ProgressBars do not scale
  down to 2dp.

- Simplify the mini-player layout by using RelativeLayout instead of
  LinearLayout.