properties: fix instantiation from control-center (bgo #720319) (rh #1040924)

->get_widget() was returning a widget that already had a parent
instead of a stand-alone widget which we could pack as we pleased.

This fixes the IPSec VPN dialogue in gnome-control-center's Network
panel not working.

See https://bugzilla.redhat.com/show_bug.cgi?id=1040924

https://bugzilla.gnome.org/show_bug.cgi?id=720319