Improved SpotLightHelper

`SpotLightHelper` geometry has been simplified. It is now just a cone. (I took the liberty of removing the sphere at the tip of the cone as it was not needed -- the light cone is sufficient.)

`SpotLightHelper` now works with spotlights that have parents.

As in the previous version, `SpotLightHelper.update()` must be called inside the render loop.

As in the previous version, an instance of `SpotLightHelper` must be a child of the scene. (This is because `SpotLightHelper.update()` relies on `Object3D.lookAt()`, which does not have hierarchical support.)