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

Move from db/DatabaseGlue.cxx, eliminating global variable.

Reduce global variable usage, move to frontend code.

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

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Publish the Client API, preparing to move more code into the Client
class.

Replaces GLib's GError.

Reduce access to the global variable "g_playlist".

.. instead of rolling own classes.