bug#28182: maybe implement CTRL++ to zoom text

[Drew Adams]

> [[[ To any NSA and FBI agents reading my email: please consider    ]]]
> [[[ whether defending the US Constitution against all enemies,     ]]]
> [[[ foreign or domestic, requires you to follow Snowden's example. ]]]
> 
>   > Did you try "C-x C-+"?
> 
> I don't think it would occur to people to try that key sequence.
> It's not what other programs use.
> 
> How about if we make C-+ and C-- do this, as they do in many
> other programs?

FWIW -

1. C-- (where it is supported) is the same as M-- and C-M--
(`negative-argument').

2. Lots of users no doubt bind C-+ (and C-=, because = is the
unshifted + key on many keyboards) for something else.

For example, I use C-= as a prefix key for comparison operations
(e.g. C-= b is ediff-buffers, C-= d is diff, C-= e is ediff-files,
and C-= w is compare-windows).  I've suggested that Emacs do the
same out of the box, = being a good mnemonic for comparison.

(Yes, I know that existing user bindings don't count for much.
Just mentioning this.)

3. If we were to bind C-- and C-+ to as you suggest then we should
at least keep the C-x C-- and C-x C-+ (and C-x C-=) bindings as well.