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.
Make URL checker optionally report bad URL value.
Trim URLs to 50 chars in msg from URL checker.
Added ImageCandidateURL.
Use ImageCandidateURL when checking srcset URLs.
Checkpointing srcset datatype checking.
Checkpointing srcset. Ugly but works now (mostly).
Cleanup (no need to valueOf() a CharSequence...).
Streamlined srcset code a bit.
Minor cleanup.
Refine srcset URL trimming & error msg formatting