Object visibility control.

Hello,

I have added two additional services to the simulation: 

- The first returns a non-flat structure of all of the blender objects in the scene. It maintains the parenting set in the blender model file. 
- The second allows an object and (optionally) its children to be hidden from view in the simulation. 

I have also added a small GUI for calling these services to control object visibility. I am using it to hide floor levels in a simulation of a multi-floor building, making it much easier to observe the simulated robots activity without having to move the FP camera so much.

Chris