GstEnginePipeline: actually perform (EBU R 128) loudness normalization

The magic: if EBU R 128 loudness normalization is enabled,
just insert `volume` GST element into the pipeline
(where ReplayGain would be inserted) and configure it.

We currently don't support changing said gain after the pipeline
was created. We might need to, though, for a number of reasons.

src/engine/gstenginepipeline.h

@@ -301,6 +301,7 @@ class GstEnginePipeline : public QObject {
   GstElement *audiosink_;
   GstElement *audioqueue_;
   GstElement *audioqueueconverter_;
+  GstElement *ebur128_volume_;
   GstElement *volume_;
   GstElement *volume_sw_;
   GstElement *volume_fading_;