Add editor setting: Manipulator Gizmo Opacity

Sometimes it might be difficult to tell where's exactly the gizmo
![](http://i60.tinypic.com/nfl0kg.jpg)

New Manipulator Gizmo Opacity setting:
![](http://i62.tinypic.com/15wm7tz.jpg)

Set Manipulator Gizmo Opacity  to 0.8:
![](http://i58.tinypic.com/24euck8.jpg)