Add feature to undo last project tree move

nw/constants/constants.py

@@ -56,6 +56,9 @@ class nwLists():
     # Item classes where the full list of novel layouts are allowed
     CLS_NOVEL = {nwItemClass.NOVEL, nwItemClass.ARCHIVE}
 
+    # Item classes which do not require items to have same class
+    FREE_CLASS = {nwItemClass.ARCHIVE, nwItemClass.TRASH}
+
 # END Class nwLists
 
 class nwRegEx():