Modeled after std::filesystem::operator/() from C++17.

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

Eliminate duplicate variable.

Move call from Mapper.cxx.

Move call from Mapper.cxx.

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.

The UPnP database plugin can now show relative song URIs for remote
songs.

Prepare for UPnP songs that retain there database identity.

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.