filter/Internal: rename struct AudioOutput to FilteredAudioOutput

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

src/output/plugins/PulseOutputPlugin.cxx

@@ -47,7 +47,7 @@
 class PulseOutput {
 	friend struct AudioOutputWrapper<PulseOutput>;
 
-	AudioOutput base;
+	FilteredAudioOutput base;
 
 	const char *name;
 	const char *server;