Don't select newly created regions after splitting selected regions (except on Mixbus)

I can't see a reason to select the newly-created regions after
splitting selected regions: I can't think of any case where you'd want
the next operation to apply to both the regions, and it makes it very
easy (in my experience) to trim/move/&c. both regions afterwards by
mistake.

However, this behaviour is desired for Mixbus, so make the selection of
regions after a split of selected regions dependent on
Profile->get_mixbus() for now.