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 use IgnoreError() when there's an overload that does not try to
give us one.

Decouple some more from GLib.

Allows forward-declaration.

Replaces struct playlist_provider.

Replaces GLib's GError.

Use the C++ API.

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

Merge locate_song_search() and locate_song_match().

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.

Improve error reporting and handling.  command.c gets the new function
print_error(), which sends a GError to the client.

Reduce header dependencies.

Optionally allow all local files.

"Insecure" mode is used for printing playlists.