Implement the second part of https://bugs.musicpd.org/view.php?id=3990

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

handle_tagtypes() is also being moved to ClientCommands.cxx, because
that command will be extended to access client-specific settings.

Move code from ReplayGainConfig.cxx.

Move code from replay_gain_get_real_mode().

Prepare full C++ exception support in the player thread.

Previously the id was being returned only when a position wasn't
specified.

playlist_file_print() already handles stored playlists.  spl_print()
is never called with a successful result.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().