This fixes another deadlock calling send_change with the
Playlist's RegionWriteLock held.
In this case due to "MIDI region copies are independent"
when duplicating MIDI regions.
```
ARDOUR::Region::send_change ()
PBD::Stateful::apply_changes ()
ARDOUR::RegionFactory::create ()
ARDOUR::RegionFactory::create ()
ARDOUR::MidiRegion::clone ()
ARDOUR::RegionFactory::create ()
ARDOUR::Playlist::duplicate ()
```