output/Interface: define a new struct AudioOutput

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

src/output/plugins/httpd/HttpdOutputPlugin.cxx

@@ -51,7 +51,7 @@ const Domain httpd_output_domain("httpd_output");
 inline
 HttpdOutput::HttpdOutput(EventLoop &_loop, const ConfigBlock &block)
 	:ServerSocket(_loop), DeferredMonitor(_loop),
-	 base(httpd_output_plugin, block),
+	 base(httpd_output_plugin),
 	 encoder(nullptr), unflushed_input(0),
 	 metadata(nullptr)
 {