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>
(cherry picked from commit a6090ebe23)