Merge pull request #250 from Unity-Technologies/fix_alignment_issue

expose onUpdate to editor panel

Author: zhuxingwei

com.unity.uiwidgets/Editor/UIWidgetsEditorPanel.cs

@@ -30,7 +30,12 @@ float _currentDevicePixelRatio {
                 return devicePixelRatioEditorOnlyOverride ?? EditorGUIUtility.pixelsPerPoint; }
         }
 
+        protected virtual void OnUpdate() {
+            
+        }
+
         void Update() {
+            OnUpdate();
             _wrapper.onEditorUpdate();
         }