The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Replaces g_str_has_suffix().

Playlist file name first, to be consistent with the other functions in
this library.

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.

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.

Try map_fs_to_utf8() first, and fall back to Path::ToUTF8() for
absolute paths.

Consistently use std::string in LoadPlaylistFile().

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

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.