forked MediaInput into two derived classes for video and audio

Rather than convoluting the MediaInput node, te MediaInput is now an abstract
base class that provides access to a Decoder and derived classes are
responsible for handling it

app/node/input/media/audio/audio.cpp

@@ -0,0 +1,26 @@
+#include "audio.h"
+
+AudioInput::AudioInput()
+{
+
+}
+
+QString AudioInput::Name()
+{
+  return tr("Audio Input");
+}
+
+QString AudioInput::id()
+{
+  return "org.olivevideoeditor.Olive.audioinput";
+}
+
+QString AudioInput::Category()
+{
+  return tr("Input");
+}
+
+QString AudioInput::Description()
+{
+  return tr("Import an audio footage stream.");
+}