network: Update VPN section

The last remaining network device to be updated is
the VPN device, and this patch is the result of this
effort.

The changes were mostly towards cleaning up and
removing unecessary code. By removing the info labels,
many getters were removed as well.

In order to achieve a listbox-like UI, a couple of
UI refactorings.

https://bugzilla.gnome.org/show_bug.cgi?id=785581