add new class RobotLinkHighlighter
add new class RobotLinkHighlighter

this makes it easy to highlight a link with a custom color, then
dehighlight the link without having to specify the original color
and always respects the robot's current alpha value.

rewrite the foot contact and fall detector to use the new highlight class

edowson/director,gizatt/director,RussTedrake/director,RobotLocomotion/director,openhumanoids/director,manuelli/director,gizatt/director,edowson/director,RussTedrake/director,manuelli/director,RussTedrake/director,rdeits/director,mitdrc/director,RobotLocomotion/director,RobotLocomotion/director,patmarion/director,patmarion/director,openhumanoids/director,manuelli/director,empireryan/director,empireryan/director,mithrandir123/director,openhumanoids/director,rdeits/director,RobotLocomotion/director,gizatt/director,openhumanoids/director,mitdrc/director,rdeits/director,edowson/director,mitdrc/director,gizatt/director,openhumanoids/director,patmarion/director,empireryan/director,edowson/director,rdeits/director,empireryan/director,gizatt/director,mitdrc/director,mithrandir123/director,rdeits/director,empireryan/director,RussTedrake/director,edowson/director,patmarion/director,mithrandir123/director,mitdrc/director,mithrandir123/director,RussTedrake/director,mithrandir123/director,RobotLocomotion/director,manuelli/director,patmarion/director,manuelli/director