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

[Drew Adams]

> All I know is we were taught to use "C-u -" for negative arguments,

Don't follow everything you are taught. ;-)

> therefore maybe C-- can be used to unzoom. And C-+ (C-=) is unbound at
> the moment.

They are free, but there is no reason to waste them.

Leaving these commands on the C-x prefix makes sense.  When the
last key is repeated the action is repeated, which is very quick.