bug#43866: 26.3; italian postfix additions

[Juri Linkov]

> May I suggest that we use a different key for this?
>
> A while back, RMS suggested that we could bind `C-+' to
> text-scale-adjust even if we can't bind `C-'.  I was not super
> enthusiastic about this at the time, but perhaps that idea is the least
> bad option.
>
> One could imagine that in combination with, for example, optionally
> binding the numerical prefix argument only to `M--'.  We could perhaps
> then consider enabling that in the "beginner friendly profile" we have
> been discussing on emacs-devel (but that no one has yet seriously worked
> on).

Ah, C-+ could be suitable for a beginner profile indeed.

What key other programs use for a Compose-like Multi_key?

https://help.ubuntu.com/community/ComposeKey
says that the default Compose Multi_key is Shift+AltGr,
and the Unicode composition key is Shift+Ctrl+U.
And indeed Shift+Ctrl+U works in all applications and xterm,
but not in Emacs on X (in Emacs on tty Shift+Ctrl+U works).

And here is more information for different systems:
https://en.wikipedia.org/wiki/Compose_key
https://en.wikipedia.org/wiki/Unicode_input