Define getIDList on UnsavedRelationList

This is to fix a bug that caused CheckboxSetFields to throw an error
when trying to call this function when editing a new DataObject. This
occurred when using the advancedworkflow module.

Thanks to simonwelsh for the majority of the work on this fix.
Merge pull request #1258 from robert-h-curry/unsavedrelationlist-getidlist

Define getIDList on UnsavedRelationList