expose onUpdate to editor panel

com.unity.uiwidgets/Editor/UIWidgetsEditorPanel.cs

                 return devicePixelRatioEditorOnlyOverride ?? EditorGUIUtility.pixelsPerPoint; }
         }
 
+        protected virtual void OnUpdate() {
+            
+        }
+
+            OnUpdate();
             _wrapper.onEditorUpdate();
         }