audio: add "dbus" audio backend

Add a new -audio backend that accepts D-Bus clients/listeners to handle
playback & recording, to be exported via the -display dbus.

Example usage:
-audiodev dbus,in.mixing-engine=off,out.mixing-engine=off,id=dbus
-display dbus,audiodev=dbus

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

audio/meson.build

@@ -26,4 +26,10 @@ foreach m : [
   endif
 endforeach
 
+if dbus_display
+    module_ss = ss.source_set()
+    module_ss.add(when: gio, if_true: files('dbusaudio.c'))
+    audio_modules += {'dbus': module_ss}
+endif
+
 modules += {'audio': audio_modules}