DERBY-6321: NetBeans project file: add XML api to source classpath

Patch derby-6321: Now that the Derby source code has been modernized
on trunk, there are less red squiggles seen in the NetBeans project
view: the old mix of Java source level versions in different parts of
Derby made the IDE confused.

There are still a few remaining red warning flags to be seen: one is
in classes that reference the XML api.  Adding
"${project.dir}/tools/java/xml-apis.jar" to the project file's source
classpath gets rid of those warnings.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1519006 13f79535-47bb-0310-9956-ffa450edef68