wwan: make sure MM1 object signals are disconnected (rh #1083727)

Otherwise if something else holds a reference to info->mm_modem, we
could end up triggering signal handlers after we've disposed of
'info'.
applet/editor: set max-width-chars on 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.

Noticed by Michael Biebl

GTK+ 3.10 hack:
https://git.gnome.org/browse/gtk+/commit/?h=gtk-3-10&id=2436627eb4644234a9e577538ec334d224e3d2be
mobile-wizard: advance from Country page when Enter is pressed (bgo #663774) (lp:981472)

Patch by Tiago Porangaba

https://bugzilla.gnome.org/show_bug.cgi?id=663774