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.

Allow printing remote playlists with the commands "listplaylist" and
"listplaylistinfo".