PluginUnconfigured exceptions are logged with level "info" instead of
"error".  This suppresses some rather boring messages in the default
log level.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/565

These messages can be confusing, but they are not critical.

This is the documented value, but for unknown reasons, "info" was
really the default.

This was never noticed because there are only very few "info" level
messages.

Make LogError()/FormatError() wrappers for those.  Now we can log
exceptions with a lower level.

Prepare for templated functions.

Print all nested exceptions on a single line to avoid confusion.

Propagate the error to the caller instead of logging it.

Preparing to move logger calls out of lower-level libaries, and
propagating error details to the caller instead.

Regression by commit bbdf2dcf

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.