properties: update .ui file to GTK 3.4+

We already require GTK 3.4+ so update the .ui file to convert
deprecated widgets (GtkVBox, GtkTable) to their non-deprecated
equivalents (GtkBox, GtkGrid).
vpnc: add support to change interface name

By default interface name is 'tun' with an incrementing number (tun0,
tun1, ...). By specifying 'Interface name' in vpnc config you can change
the name to something more descriptive.