Some appdata.xml fixes.

* Add name/summary.
* Set metadata_license to CC-BY-SA-3.0.
* Set project_license.
* Add screenshot caption.
* Use component rather than application.
Remove unused files.
Fix AutoDJ empty-queue disabling to update the CO and disconnect signals.
Fix race condition in starting AutoDJ

* Better support for multiple decks: switch from loadTrack signal (which has an
  implicit target) to loadTrackToPlayer so we are always explicit about
  what player to load a track into.
* When first loading AutoDJ, use load-and-play signal instead of
  assuming the track is loaded and ready to play when we call play().
* Make the no-deck-playing enable AutoDJ test more complete to show we
  properly transition into ADJ_IDLE mode.
Fix race condition between loading a track and removing it from the
AutoDJ queue. Potential fix for Bug #1206080.
Merge remote-tracking branch 'upstream/master' into deere_updates