Add ability to enter new collection UDFs and edit saved collection UDFs

A tag is a monotonically increasing identifier set by the UDFValueField to identify it.  It gets sent to the UDFCollectionEditActivity when you edit an Alert, and then returned back to the field group, so the field group knows which field to replace.

As I originally wrote it, the field group thought all edits were adds, and made a new field every time you edited an alert.

There should be some comments about the tag in UDFCollectionValueField as well as in UDFCollectionActivity's receiveActivityResult method