Eliminate duplicate variable.

Replaces map_to_relative_path() from Mapper.cxx.

Use Storage::MapUTF8() internally, don't use global variables.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Make the DetachedSong(Song) conversion constructor private.  Everybody
should use map_song_detach() which will take over more
responsibilities soon.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

If no music_directory is configured, return nullptr instead of an
empty string.  This fixes a crash when db_file is configured without
music_directory.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Avoid the conversion to an allocated char*, let the caller decide.

Replaces GLib's GError.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

Duplicate the music_dir variable: one encoded in UTF-8, and another
one using the configured filesystem character set.  This fixes an
ancient MPD bug.

Remove GLib dependency from some headers.

Shortcut for map_directory_fs(db_get_root()).

If a song with an absolute path points inside the music directory,
print only the relative part.  This happens when partial songs from a
playlist file were loaded.

This fixes an inconsistency in the stored playlist subsystem: when
obtaining the list of playlists (listplaylist, listplaylistinfo), the
file names in the playlist directory are converted to UTF-8 (according
to filesystem_charset), but when saving or loading playlists, the
filesystem_charset setting was ignored.

Added another glue function in main().