[hr] Bug 177582 - Add Framework action removes the content of web.xml

https://netbeans.org/bugzilla/show_bug.cgi?id=177582

Problem: Adding a web framework to Web Project overwrites existing 
web.xml with a new empty one and user loses whatever was previously there.

Cause: since Java EE 6 web.xml is optional but most of the existing 
frameworks require it and web.xml has to be created if it does not 
exist. There is error in current code which creates web.xml always 
regardless if it already exists or not.

Fix: trivial - skip web.xml creation if it already exists. To prevent 
similar issues in future I enhanced semi-API method which creates 
web.xml to check file's existence and throw IllegalStateException in 
case it does.

Change was reviewed and is going to be tested today. I'm not sure 
whether there is time to include it in RC1, but if there are no 
objections it can be perhaps integrated to RC2.

Thanks,
-David