bug#27530: patch to cut and copy secondary

[Tak Kunihiro]

>> The function exchanges point and mark.
>> When there is region, this also creates secondary.
> 
> Why?  What if I want to just exchange point and mark,
> and not also copy the region to the secondary?

I proposed to put a function in lisp/mouse.el that can be remapped to
C-x C-x by an user.  I did not propose to replace
exchange-point-and-mark.

I create the secondary selection every wheel scroll as memory of
region.  To me, C-x C-x works well to recall region.