JDK-8010495: Update JAXP NetBeans project - add support for generating javadoc

[Daniel Fuchs]

Hi guys,

I'd like to propose a small change to jaxp/build.xml and
jaxp/nbproject/project.xml - in order to allow editing
JAXP sources in NetBeans, as well as generating the JAXP
javadoc for previewing purposes.

I actually stole the javadoc target from
jdk/make/netbeans/common/shared.xml

The changes are small & targeted at developers.
There should be no impact on the build process.

Here is the webrev:

http://cr.openjdk.java.net/~dfuchs/JDK-8010495/jdk8/webrev.00/

best regards,

-- daniel