output/Internal: move thread-specific stuff to AudioOutputControl

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

test/run_output.cxx

@@ -43,8 +43,6 @@
 #include <stdlib.h>
 #include <stdio.h>
 
-void AudioOutput::Task() {}
-
 class DummyAudioOutputClient final : public AudioOutputClient {
 public:
 	/* virtual methods from AudioOutputClient */