Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

This adds support for the full set of MPD 0.21 filter types.

That way, each plugin can decide to implement it better.

No undefined behavior.

Requires libmpdclient 2.15.

Prepare to allow more complex expressions.

This enforces the "not nullptr" rule.

Closes #284

Closes #283

Closes #258

Fixes "search already in progress" errors.

Allows database plugins to use the EventThread, e.g. for CURL
integration.

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

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

If the base class is not accessible, the "catching" the base class
won't work.  This caused the fatal error:

 terminate called after throwing an instance of 'LibmpdclientError'