Play loop, change loop-location, undo.
Undo calls Locations::set_state, takes a writer-lock,
and calls Location::set_state which emits a Changed signal.
This triggers Editor::location_changed, and if loop-location
changed while looping, update_loop_range_view() queries the
loop location, taking a reader-lock.
This leads to a recursive lock, RWLock::ReaderLock after
a RWLock::WriterLock does not cause a deadlock, however
releasing the ReaderLock effectively also unlocks the WriterLock.
This leads to a deadlock next time a writer-lock is acquired.
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