output: added audio_output_get_name()

Reduce direct accesses to the audio_output struct from the plugins:
this time, eliminate all accesses to audio_output.name.  The name is
required by some plugins for log messages.

src/output_api.h

@@ -99,4 +99,6 @@ struct audio_output {
 
 extern struct notify audio_output_client_notify;
 
+const char *audio_output_get_name(const struct audio_output *ao);
+
 #endif