NetBeans does not recognize latest sun-*.xml files

This is actually captured in two issues:

https://netbeans.org/bugzilla/show_bug.cgi?id=177307 and 
https://netbeans.org/bugzilla/show_bug.cgi?id=177939

The root cause of issue 177307 was a change in the public id of the 
sun-web-app_6_0-0.dtd.

The code to support the other sun-*.xml files had not been developed.

The user would see an exception in the dev builds.  When I tested this 
in a 6.8 rc environment, the multiview editors for all the various 
sun-*.xml files would be blank, except for the XML view.

The change is safe for a couple reasons:

1. it is difficult to run into the issue, since 
https://netbeans.org/bugzilla/show_bug.cgi?id=177943 prevents users from 
generating a sun-*.xml file that includes the newest DOCTYPE info.

2. The code is mostly boilerplate.

3. have integrated unit test that covers the code that is the root cause 
of 177307 and 177939.

See the issues for notes on review and testing.

The issues also have links to the changes.

Thanks,
vbk