AutoDJ fixes.

* Remove QSignalMapper usage and instead proxy signals from
  DeckAttributes.
* Connect track load / unload / failed-to-load signals from
  BaseTrackPlayer. Lays the groundwork for solving Bug #1396931.
* Add a helpful debug flag for tracing AutoDJ calls.
* Remove PlayerInfo usage from AutoDJ for getting the loaded
  track. Instead, use BaseTrackPlayer itself.
It could be a nice shortcut -- but I really value being explicit over implicit. What particularly bothers me is automatic conversions of pointers to boolean.