sinceVersion on nested types within a composite

[mjpt777]

The XSD for 1.0 allows for the sinceVersion attribute to be set on nested types within a composite. This is contrary to the spec as it does not state how composites can be extended.

[donmendelson]

It seems we need a different XML type for a simple type that is standalone versus one that is contained by a composite type. Another difference is that the standalone would not have an offset attribute.