bug#27530: patch to cut and copy secondary

[Tak Kunihiro]

>> I found that the exchange of primary and secondary can be implemented
>> into C-x C-x as `mouse-exchange-point-and-mark-secondary'.
>> 
>>   (global-set-key [remap exchange-point-and-mark] 
>> 'mouse-exchange-point-and-mark-secondary)
> 
> Hmm...  In Emacs 24, we have changed how selection works to conform
> with other modern X applications, by having primary selection set only
> via mouse gestures, whereas keyboard commands work with the clipboard.
> Doesn't your proposal go in the opposite direction?

I see.  I wonder if the direction lets the secondary selection react
with region-reacting functions such for comment-dwim.

> In any case, a command whose name starts with "mouse-" should have the
> "e" interactive spec, and should react to mouse gestures, not to
> keyboard keys.  I also don't like advising users to remap "C-x C-x" to
> anything that smells a mouse.

I see.  I will look for somewhere else to put in.