Defer EditorInspector::update_tree to the process stage to improve performance

`EditorInspector::update_tree` is expensive, so defer the call to the process phase
to prevent multiple calls in a single frame (when switching scene tabs).

editor/editor_inspector.h

@@ -540,6 +540,8 @@ class EditorInspector : public ScrollContainer {
 	void _show_add_meta_dialog();
 	void _check_meta_name(const String &p_name);
 
+	void _update_tree();
+
 protected:
 	static void _bind_methods();
 	void _notification(int p_what);