Various playlist operations can change region-properties
which results in Region::send_change being emitted while
the Playlist::RegionWriteLock is held.
This can result in recursive lock and/or deadlocks or crashes. e.g.
Insert time -> Playlist::shift -> Region::RegionPropertyChanged
-> EditorSummary::set_background_dirty -> Editor::session_gui_extents
-> Playlist::get_extent -> read-lock is taken after write-lock.
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