This allows to properly toggle "Glue to Bars/Beats".
Editor::toggle_region_lock_style uses Region::position_time_domain(),
However Region::set_position_time_domain() checked the duration's
time-domain.
Furthermore timecnt_t::set_time_domain() changes both the
position and the duration's time domain. This can lead to
various issues. We only need to change the time-domain of
the timepos_t _position.
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