Change the selected extruder for all child nodes of a group

Instead of setting the extruder for the group node, go through all
children and set their extruders instead. Fixes extruder selection on
groups.

Contributes to CURA-3609

cura/CuraActions.py

@@ -9,6 +9,7 @@ from UM.Event import CallFunctionEvent
 from UM.Application import Application
 from UM.Math.Vector import Vector
 from UM.Scene.Selection import Selection
+from UM.Scene.Iterator.BreadthFirstIterator import BreadthFirstIterator
 from UM.Operations.GroupedOperation import GroupedOperation
 from UM.Operations.RemoveSceneNodeOperation import RemoveSceneNodeOperation
 from UM.Operations.SetTransformOperation import SetTransformOperation
@@ -81,9 +82,31 @@ class CuraActions(QObject):
     @pyqtSlot(str)
     def setExtruderForSelection(self, extruder_id: str) -> None:
         operation = GroupedOperation()
+
+        nodes_to_change = []
         for node in Selection.getAllSelectedObjects():
+            # Do not change any nodes that already have the right extruder set.
             if node.callDecoration("getActiveExtruder") == extruder_id:
                 continue
+
+            # If the node is a group, apply the active extruder to all children of the group.
+            if node.callDecoration("isGroup"):
+                for grouped_node in BreadthFirstIterator(node):
+                    if grouped_node.callDecoration("getActiveExtruder") == extruder_id:
+                        continue
+
+                    if grouped_node.callDecoration("isGroup"):
+                        continue
+
+                    nodes_to_change.append(grouped_node)
+                continue
+
+            nodes_to_change.append(node)
+
+        if not nodes_to_change:
+            return
+
+        for node in nodes_to_change:
             operation.addOperation(SetObjectExtruderOperation(node, extruder_id))
         operation.push()