subtyping uima.cas.String

[Philip Ogren]

I was just putting some unit tests together and was editing a type 
system and noticed that I can't seem to subtype a type that is a subtype 
of uima.cas.String in the CDE.  I thought this was maybe a bug that I 
should bring attention to.  For my part, I'm not concerned with the 
ability to create nested subtypes of uima.cas.String - so apologies if 
this is noise.