InstrumentTrack: presetPreviewPlayHandle no longer a friend class

For a long time presetPreviewPlayHandle has been a friend class of
InstrumentTrack as it had to modify MidiPort settings. However the
InstrumentTrack's MidiPort is now accessible via getMidiPort() so
there's no need for keeping presetPreviewPlayHandle a friend class
anymore.

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

src/core/preset_preview_play_handle.cpp

@@ -151,8 +151,8 @@ presetPreviewPlayHandle::presetPreviewPlayHandle( const QString & _preset_file,
 
 	// make sure, our preset-preview-track does not appear in any MIDI-
 	// devices list, so just disable receiving/sending MIDI-events at all
-	s_previewTC->previewInstrumentTrack()->m_midiPort.setMode(
-							midiPort::Disabled );
+	s_previewTC->previewInstrumentTrack()->
+				getMidiPort()->setMode( midiPort::Disabled );
 
 	// create note-play-handle for it
 	m_previewNote = new notePlayHandle(