We were using g_utf8_strlen() to measure strings for layout, but that
counts combining marks too, and also fails to deal with "fullwidth"
(ie, double-width) CJK characters.
Add a new utility function to do a better job of this (based on code
from vte), and use it everywhere.
"Cancel" didn't really make sense here, but changing it to "Quit" is
also weird because if you were going to use the "OK" button you'd have
to tab past "Quit" first... So make it be an option in the menu
instead.
