Be consistent.

Use a listener interface instead of GlobalEvents.

Make it "const".

Use the existing PcmDither library.

Wake up the OutputThread only if it hasn't already been woken up and
if it isn't already in the playback loop.

Replaces GLib's GError.

Implements Mantis ticket 0003340.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

The output thread could hang indefinitely after finishing CANCEL,
because it could have missed the signal while the output was not
unlocked in ao_command_finished().

This patch removes the wait() call after CANCEL, and adds the flag
"allow_play" instead.  While this flag is set, playback is skipped.
With this flag, there will not be any excess wait() call after the
pipe has been cleared.

This patch fixes a bug that causes mpd to discontinue playback after
seeking, due to the race condition described above.

.. and destruct the output object properly in test/run_output.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.