libnm-gtk: set max-width-chars on WWAN wizard labels to prevent wide dialogs

GTK+ 3.10 and lower capped labels with wrapping set, but without
max-width-chars set, from taking up the entire screen width.  GTK+
git master no longer does that, so some of our labels never wrap
and the dialogs look ugly.

GTK+ 3.10 hack:
https://git.gnome.org/browse/gtk+/commit/?h=gtk-3-10&id=2436627eb4644234a9e577538ec334d224e3d2be