aout: add distinct start/stop callbacks (refs #4787, refs #7601)

aout: add distinct start/stop callbacks (refs #4787, refs #7601)

This would enable the input manager to hold an audio output plugin at
all times. Then it would be possible to keep track of output devices,
volume, policy and (JACK) routing continuously. For this to work,
outputs will need to be patched so that device selection and volume
handling works even when playing back.

Note though that this commit only introduces the change in the
audio_output_t interface; it does not implement the features above.