Since LIBDIR of gnome-control-center and the VPN plugins aren't
necessarily the same, use the paths as specified by the VPN plugins and
only reconstruct the path if it is not absolute or we fail to load the
plugin.
Patch from Debian package, by Michael Biebl <biebl@debian.org>
https://bugzilla.gnome.org/show_bug.cgi?id=764509
Unfortunately, the VPN plugins provide their own .ui files for their
editor pages, so we can't make them look competely GNOME-3-ish. But
the code does try to fix them up a little bit by realigning the
labels.
vpn-helpers.[ch] is nearly identical to network-manager-applet's,
but eventually this code will move into libnm-gtk.
https://bugzilla.gnome.org/show_bug.cgi?id=691285