Move some SDK global properties and types into namespaces.

[ctrlaltdavid]

In order to reduce the amount of pollution in the namespace and tidy the use docs. Breaking change and will improve the foundation for further work so do as soon as reasonably possible.

Entities namespace:

• EntityType
• EntityProperties, ModelEntityProperties, etc. and related.
• Shape and rename to shape. Resolve w.r.t. ShapeType.
• etc.

Skeleton namespace:

• BoneType
• SkeletonJoint
• etc.

AudioMixer:

• AudioPositionGetter.