connection-editor: rename new-connection.c to connection-helpers.c

Originally this file only had the new_connection* stuff in it, but
later it got delete_connection() too and was supposed to have been
renamed (the multiple-include guard in the header was already
"__CONNECTION_HELPERS_H__"), but apparently I forgot to rename it...
connection-editor: factor out the code for deciding ip4/ip6 support
connection-editor: allow IP4 "Disabled" method for more types

Allow disabling IPv4 on any type that supports IPv6.
connection-editor: support IPv6 on mobile devices

https://bugzilla.gnome.org/show_bug.cgi?id=733713
editor: remove "(PPP)" from WWAN addressing modes

Most WWAN devices don't use PPP for addressing these days.
connection-editor: support IPv6 on mobile devices (bgo #733713)