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.

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'

SimpleDatabase::Visit() always throws exception when visiting a song.
Adding a song to the queue always results in a "No such directory"
error response, although the song is successfully added.  This
behaviour was introduced by commit
3ff728ab.  This patch fixes this.

https://bugs.musicpd.org/view.php?id=4601

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Implementations shall use exceptions instead.