ResourceBrowser: added a PianoView widget

Added a PianoView widget to ResourceBrowser so one can easily preview selected
preset, sample etc.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>

include/ResourcePreviewer.h

@@ -28,6 +28,7 @@
 
 #include "track_container.h"
 
+class Piano;
 class ResourceItem;
 class instrumentTrack;
 
@@ -41,6 +42,8 @@ public:
 	void preview( ResourceItem * _item );
 	void stopPreview();
 
+	Piano * pianoModel();
+
 
 private:
 	class PreviewTrackContainer : public trackContainer