Fix breakage from commit bea5681f

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

The old struct config_param remains only for top-level string options.

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Replaces GLib's GError.

Use these instead of exposing the internal roar_t struct.

Should fix build errors for Win32.