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.

Don't pass a GError** to g_key_file_get_X().  We don't need to dispose
something we didn't request in the first place.

Remove the forward_list::reverse() call.  It was not necessary,
because pls_parser() already reads the playlist in reverse order.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces struct playlist_provider.

Replaces GLib's GError.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The caller is responsible for verifying the song URI.