Add editor setting: Manipulator Gizmo Opacity

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