Keep track of whether there is data being played, and don't call
AudioOutput::Drain() after Cancel() has been called already.

Fixes #994 - moveoutput: new AudioOutputControl created from copyMoving an output to a partition is now done via MultipleOutputs::AddCopy(),using a new AudioOutputControl constructor. Tags and always_on settings willpersist when moving outputs between partitions.

This allows interrupting the output thread (for some plugins which
implement this method).  This way, operations can be canceled
properly, instead of waiting for some external entity.

The former is deprecated by C++14. The standard says they are the same:

The header defines all types and macros the same as the C standard library
header<stdint.h>.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-const-return-type
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Prepare the transition to std::mutex.

Instead of keeping all open outputs locked, let ClearTailChunk() stall
playback until MultipleOutputs::CheckPipe() has updated the MusicPipe.

With the new command, the decision to pause or close the output moves
into the output thread.

Prepare for adding another Cond attribute.

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

struct AudioOutput should not know or care about multi-threading.