Add a config option to control region selection after split.

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.