Remove unnecessary load_atomic.
AutoDJ Refactor

Pulls all processing logic out of DlgAutoDJ into AutoDJProcessor
class. This provides a couple of nice fixes:

* Multiple DlgAutoDJ instances can exist at once. (potentially necessary
  for advanced skin usage).
* AutoDJ state isn't lost when reloading or switching skins. (Bug
  #976704)
* Logic is separate from user interface -- AutoDJProcessor is much more
  unit testable.
Move src/library/autodjfeature.(h/cpp) to src/library/autodj/.
Add some basic unit tests for AutoDJProcessor.