Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Look up top-level config options by enum (= integer), not by name
string.

Implements Mantis ticket 0003340.

No, really!  This fixes a regression of commit 74617389, which
changed the order of filter plugins.

Don't require the caller to provide them.

If output plugin fails to init it will try to call ao_base_finish() immediately,
which segfaults because replay gain filters are not initialized yet and contain
garbage values.

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.

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.

Don't add it to the filter chain, because we need to apply replay gain
before cross-fading with the next song.  Add a second replay_gain
filter which is used for the song being faded in (chunk->other).

Remove cross_fade_apply(), and call pcm_mix() in the output thread,
mixing the chunk and chunk->other together.

Did you ever accidently click "stop" while feeding a radio station?
This option sets the output device to "pause" to disable the "close"
method.  It falls back to "pause" then, which is specific to the
plugin.  Some plugins implement it by feeding silence.