Fix how dataflows corrupt when referenced dataflow is deleted

If I delete a dataflow I think is unused, but it is actually referenced in another dataflow--that second dataflow will become unable to be opened or edited, corrupted with no option but to delete it. Example: I have a dataflow named "US Contacts-OLD" that is appended with other similar dataflow entities in a "All Contacts" dataflow. If I delete "US Contacts-OLD" but forget to remove the reference in "All Contacts"--I will have to delete and recreate "All Contacts". Any dataflows referencing "All Contacts" will have to be updated with the new GUID.