[NbtkLightSwitch] add an insensitive indicator
[NbtkLightSwitch] add an insensitive indicator

Fixes bug 4601 - Make NBTKGtkLightSwitch look more distinct when
disabled

clutter-project/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,clutter-project/mx