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.
Unless NMTUI_DEBUG is set, don't pop up a dialog when a
g_warning/critical/etc occurs. Just hide the main screen briefly so
the error doesn't get printed over the UI, and restore it after; any
messages that get logged will be visible after nmtui exits.
