Nodes: Add icon and separate "New Group" operators

The current placement of the operators make it easy for them to blend in with
other group nodes in that menu.

This patch adds an icon to better indicate the nature of these operators, and
have separators between it and other entries of the menu.

Pull Request: https://projects.blender.org/blender/blender/pulls/147330

scripts/startup/bl_ui/node_add_menu.py

@@ -270,7 +270,11 @@ class NodeMenu(Menu):
     @classmethod
     def new_empty_group(cls, layout):
         """Group Node with a newly created empty group as its assigned nodetree."""
-        props = layout.operator(cls.new_empty_group_operator_id, text="New Group", text_ctxt=i18n_contexts.default)
+        props = layout.operator(
+            cls.new_empty_group_operator_id,
+            text="New Group",
+            text_ctxt=i18n_contexts.default,
+            icon='ADD')
 
         if hasattr(props, "use_transform"):
             props.use_transform = cls.use_transform
@@ -284,14 +288,14 @@ class NodeMenu(Menu):
         node_tree = space_node.edit_tree
         all_node_groups = context.blend_data.node_groups
 
+        operators = []
+        operators.append(cls.new_empty_group(layout))
+
         if node_tree in all_node_groups.values():
             layout.separator()
             cls.node_operator(layout, "NodeGroupInput")
             cls.node_operator(layout, "NodeGroupOutput")
 
-        operators = []
-        operators.append(cls.new_empty_group(layout))
-
         if node_tree:
             from nodeitems_builtins import node_tree_group_type