network: Add header to VPN section

Since each VPN will be a row in a listbox, we
can't rely on NetVPN:add_to_stack() to handle
the header.

This header must, then, be handled by the panel
itself. For now, we just open the already available
dialog to add connections, when the ideal approach
(to be implemented yet) is to move the contents
of this dialog in a built-in popover.

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