It's very convenient to use a shift-translated key to activate the region,
because e.g. 'S-C-f' requires typing less keys than 'C-SPC C-f'.

But when there is a need to extend the region by typing a navigation key
that is not shift-translated, it abruptly deactivates the region.
The current default behavior was implemented in such unusable way
because "other apps" behave the same way.

So the following patch provides an option for the behavior that is more
convenient than in "other apps":