Lower the default rain volume.

Update issue #418
Status: Fixed
I can't think of a good place to put the GUI for this, so instead I've lowered the default rain volume so you can adjust the volume of whatever else you're listening to to match.

src/engines/gstengine.cpp

@@ -745,6 +745,7 @@ int GstEngine::AddBackgroundStream(const QUrl& url) {
   if (!pipeline) {
     return -1;
   }
+  pipeline->SetVolume(30);
   // We don't want to get metadata messages or end notifications.
   disconnect(pipeline.get(), SIGNAL(MetadataFound(Engine::SimpleMetaBundle)), this, 0);
   disconnect(pipeline.get(), SIGNAL(EndOfStreamReached(bool)), this, 0);