Allow iframe@role=presentation. Thx Phil Tibbits.
Allow role=presentation for embed & object.
Added stub for "source size list" datatype.
Added stub for "image candidate strings" datatype
Minor tweek to descriptive strings.
Added VnuBadAttrValueException.
Simplified the schema parts for srcset.

The width-required-or-not datatype complexity is moving to Assertions.java.
"IRI reference" -> "URL"; "abs. IRI" -> "abs. URL"
Use .THE_INSTANCE for datatypes, not new foo().
Put srcset datatype association in Assertions.java
Checkpointing srcset datatype checking.
Minor update to sizes datatype stub.