[SharpDX] Add Up, Down, Left, Right, ForwardRH, ForwardLH, BackwardRH and BackwardLH to Vector3

I think these are useful, even with the slight added complication of having to provide LH and RH versions of Forward and Backward. 

(In theory, we should all be able to use UnitX, UnitY and UnitZ to do the same thing as these properties, but sometimes it's nice to have a helping hand.)