mixin types should not be allowed to inherit non-mixin types

Section 3.5.7.2 of the JSR-283 specification (aka, "JCR 2.0") states:

A mixin node type may have one or more supertypes, which must also be mixin types.

At the moment however, ModeShape does not perform any kind of validation in this regard, allowing a mixin type to inherit from a non-mixin type.
We should add this validation so that an error is raised during type registration if the above case occurs.