Core: Add item.filler helper (#4081)

* Add filler helper

* Update BaseClasses.py

BaseClasses.py

@@ -1262,6 +1262,10 @@ def useful(self) -> bool:
     def trap(self) -> bool:
         return ItemClassification.trap in self.classification
 
+    @property
+    def filler(self) -> bool:
+        return not (self.advancement or self.useful or self.trap)
+
     @property
     def excludable(self) -> bool:
         return not (self.advancement or self.useful)