InstrumentTrack: renamed getPiano() to pianoModel()

Renamed getPiano() to pianoModel() to better express what this
function does.

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

include/instrument_track.h

@@ -143,7 +143,7 @@ public:
 		return &m_midiPort;
 	}
 
-	Piano * getPiano( void )
+	Piano * pianoModel( void )
 	{
 		return &m_piano;
 	}