Add Instance Placeholder validation check.

Disallow toggling placeholder on editable children
or inherited nodes.

editor/scene_tree_dock.cpp

@@ -1236,6 +1236,10 @@ void SceneTreeDock::_tool_selected(int p_tool, bool p_confirm_override) {
 				break;
 			}
 
+			if (!_validate_no_foreign()) {
+				break;
+			}
+
 			List<Node *> selection = editor_selection->get_selected_node_list();
 			List<Node *>::Element *e = selection.front();
 			if (e) {