Perform Platform physics changes on scene changed again and use a custom operation for better op merging

PlatformPhysicsOperation.py

@@ -0,0 +1,27 @@
+from UM.Operations.Operation import Operation
+from UM.Operations.AddSceneNodeOperation import AddSceneNodeOperation
+from UM.Operations.TranslateOperation import TranslateOperation
+
+##  A specialised operation designed specifically to modify the previous operation.
+class PlatformPhysicsOperation(Operation):
+    def __init__(self, node, translation):
+        super().__init__()
+        self._node = node
+        self._translation = translation
+
+    def undo(self):
+        pass
+
+    def redo(self):
+        pass
+
+    def mergeWith(self, other):
+        if type(other) is AddSceneNodeOperation:
+            other._node.translate(self._translation)
+            return other
+        elif type(other) is TranslateOperation:
+            other._translation += self._translation
+            return other
+        else:
+            return False
+