Add a config option to control region selection after split.

Add a configuration variable to choose the behaviour of the region
selection after splitting selected regions.

Add options to choose between all eight possible combinations of 'existing
unmodified selected regions', 'newly-created regions to left of split',
and 'newly-created regions to right of split', but comment out all but the
three least crazy ones for now. If anyone wants them, they're there.
Implement configurable region selection after split

Implement the configured behaviour of region selection after split.
Change default for region selection after split on Mixbus

Mixbus prefers that both the newly-created regions as well as the existing
selection are selected after splitting selected regions: make this the
default on Mixbus.