Add some conveniency classes to the VR library

[codeanticode]

Right now, very basic functionality in VR such as object selection and camera movement need to be implemented manually in the sketch. The VR library should incorporate a few classes to support this functionality. Commit de881bd adds initial implementations of new classes VRSelector and VRCamera.

[codeanticode]

VRSelector not longer needed with the planned addition of the ray casting API #559, but keeping VRCamera to implement some basic utilities, such as #471