Removed hardcoded models[0] limitations.  Rejigged hierarchy bits and pieces.

Fixes #22
Added Model.Santise()  This method loops through all Meshes of a Model and removes any without geometry

Fixes #24
Added Optimise() to dedupe index and vertex buffers

Fixes #23
Reset boundingbox on scene.Reset()

Fixes #27
Fix crash caused by clicking on either Root or Scene after an asset has been loaded
ContentPipeline.Stainless.CNTExporter:  Export the right thing
Graphics.ViewportManager:  Focus is required for numpad keys to function properly and not required for d, * and /

Fixes #29