Added way to dynamicly set loading order

This is to ensure that Global stacks are loaded before extruders, so once the extruders are deseralized, they can always find the next (global) stack

CURA-3497

cura/Settings/GlobalStack.py

@@ -40,6 +40,10 @@ class GlobalStack(CuraContainerStack):
     def extruders(self) -> list:
         return self._extruders
 
+    @classmethod
+    def getLoadingPriority(cls) -> int:
+        return 2
+
     ##  Add an extruder to the list of extruders of this stack.
     #
     #   \param extruder The extruder to add.