Cua-mode binding of S-return

[storm@cua.dk (Kim F. Storm)]

Currently cua-mode uses S-return to toggle rectangle marking.

This works nicely most of the time, but it is just too easy to
accidentally hit it after another shifted key, e.g. an upper case
letter.

Therefore, I would like to move this to another binding, but I 
don't quite know which.  

C-return seems like a good candidate, analogue to C-space for the normal mark.

Unfortunately, C-return is used by tex-mode and thumbs-mode:

   (define-key map [(control return)] 'tex-feed-input)
   (define-key map [(control return)] 'thumbs-set-image-at-point-to-root-window)

Calculator also uses it, but this is in a special environment where rectangle
marking doesn't make much sense anyway:

   (calculator-save-and-quit [(control return)]

The thumbs binding is a bit odd, as it could just as well be bound to a regular
key, such as `r'.

That really leaves just the tex-mode binding -- so I wonder how commonly
used that binding is (C-c C-m also runs tex-feed-input).

Of course, this is all relevant only when you use cua-mode.

But we have talked about separating the cua-mode rectangle code
from cua, and as such a good binding to toggle rectangle marking
will have to be found anyway.

-- 
Kim F. Storm <storm@cua.dk> http://www.cua.dk