Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Avoid accessing FilteredAudioFormat attributes directly.

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

Moved code from AudioOutputControl::InternalOpen2().

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

Prepare for moving the AudioOutputSource to class AudioOutputControl.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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

Reduce overhead by eliminating MultipleOutputs::ResetReopen().

.. and remove the obsolete method OpenFilter().

Don't expose MusicChunk instances, provide higher-level access to
chunk contents.

Try harder to skip steps (reopen filter, reopen output) if the
AudioOutput is already open.