View3D: Add support for picking (mouse/touch input)

Currently there is no front-end API for handling picking in 3D scenes.  There is however a picking/ray-casting API as part of the backend.  We need to create a way to cast ray's into the scene from the front end and return a list of items that ray intersects with, as well as where on the object it intersects.  From there it should be possible to add a way to cast a ray into the scene from the perspective of the camera/view3d where a touch/mouse even was created.

We also need to define:

• What is done when something is clicked.
• Do 3D objects have event handlers (onClicked)
• How do 3D objects say they are pickable?
• What kinds of API's are needed