iso2022: Always intern the codeset name
widget: Add setting for CJK ambiguous width

When using UTF-8 encoding, normally ambiguous-width characters are narrow.
However, some uses (most importantly CJK) want them to be wide instead.

https://bugzilla.gnome.org/show_bug.cgi?id=352654
https://bugzilla.gnome.org/show_bug.cgi?id=614617