bug#43866: 26.3; italian postfix additions

[Juri Linkov]

> Itʼs already in latin-postfix and on C-x 8 * E, is that really
> necessary?

I wonder why C-x 8 provides key sequences that are not mnemonic
and so hard to remember?

Would it make sense to support exactly the same keys that are
provided by the X11 compose method?  I mean that are in the file
/usr/share/X11/locale/en_US.UTF-8/Compose
also available at
https://help.ubuntu.com/community/ComposeKey
and
https://cgit.freedesktop.org/xorg/lib/libX11/plain/nls/en_US.UTF-8/Compose.pre

For example, for every such line:

  <Multi_key> <equal> <E>      : "€"   EuroSign # EURO SIGN

replace <Multi_key> with C-x 8, and bind such key sequences:

  C-x 8 = E   => "€"

and for all other keys as well, e.g.

  C-x 8 . . . => "…" (HORIZONTAL ELLIPSIS)