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

Reduce bloat.

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.

Decouple some more from GLib.

Allows forward-declaration.

Replaces struct playlist_provider.

Allow the plugin to allocate the GetSong() return value.

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.

Reduce header dependencies.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

Optionally allow all local files.

"Insecure" mode is used for printing playlists.

Return early on error, save one level of indent.

Memory leak fix.  The input_stream object passed to
playlist_list_open_stream_suffix() must be closed by the caller - this
however never happens in playlist_list_open_path(), because it does
not return it to the caller.

Accept absolute paths if they point into the music directory.

Preparation for the next patch.