Allow forward-declaring it.

Map ID3 "TIT1" to this new core tag type.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/563

Closes https://github.com/MusicPlayerDaemon/MPD/issues/562

release v0.21.9

Closes https://github.com/MusicPlayerDaemon/MPD/issues/340

If the decoder finishes decoding the current song between the two
IsIdle() checks, MPD stops playback instead of starting the decoder
for the next song.

This is usually not visible problem, because the main thread restarts
it via playlist::ResumePlayback(), but that way it, ignores "single"
mode.

As a workaround, this commit adds another "queued" check which
re-enters the player loop and checks again whether to start the
decoder.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/556

The "queued" flag can only possibly be set if the decoder is still
decoding the current song or if the decoder is stopped.  This is also
what the following assert() checks.  This check was not necessary.

Another step towards supporting multiple readers.

There is just one thread.

The thread will always attempt to read more data since commit
2cf6b776, so we don't need to tell it
to continue.

The "seek_error" attribute will be eliminated soon.

More preparations to support multiple readers.

If a read error occurs, it is very unlikely that the InputStream will
ever recover.  Removing the code removes some code complexity which
just isn't worth it.  And it allows supporting multiple readers for
one buffer.

After seeking, a new read should be done.

Prepare to reuse it in another class.

ALSA is just one out of many output plugins, and detailed plugin
documentation should only live in the user manual, without having
duplicates in the (brief) manpage.

Also move "mixer_type" to the "optional audio output parameters"
section; it is a generic option, not specific to ALSA.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/552