Add undo_reference and undo_data methods to DbGeneric

Author: stevenyoungs

gramps/gen/db/generic.py

@@ -2648,7 +2648,19 @@ def db_has_bm_changes(self):
     def get_undodb(self):
         return self.undodb
 
-    def undo(self, update_history=True):
+    def undo_reference(self, data, handle: AnyHandle):
+        """
+        Helper method to undo a reference map entry
+        """
+        raise NotImplementedError
+
+    def undo_data(self, data, handle: AnyHandle, obj_key):
+        """
+        Helper method to undo/redo the changes made
+        """
+        raise NotImplementedError
+
+    def undo(self, update_history: bool = True):
         return self.undodb.undo(update_history)
 
     def redo(self, update_history=True):