In locales where the Wi-Fi signal-strength characters couldn't be
represented (eg, LANG=C), the entire Wi-Fi SSID + signal strength
string would fail to convert, causing the Wi-Fi section of the
connection list to show up as a series of blank lines.
Fix this by testing beforehand whether the characters can convert, and
falling back to plain ASCII if not. (And also, fix the similar code in
nmt-newt-section.c, which got broken when nmt_newt_locale_from_utf8()
was changed to never return NULL.)
Also, for paranoia, represent the signal-strength strings via \nnn
escapes rather than actual UTF-8 data, to guarantee that they get
compiled to the expected values even if the source files get
re-encoded.
https://bugzilla.gnome.org/show_bug.cgi?id=733007
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.
