[core] Adjust for API changes of newer Java versions
In newer Java versions, the type of DefaultMutableTreeNode.children is
explicitly declared as Vector<TreeNode>, hence we need to adjust the
return type of children.elements() and cast to CheckNode.
Florian Schmaus <flo@geekplace.eu>
680065fb4eba649a4bab7a49efdc6378faf3f414
[core] Explicitly specificy JAXB as dependency
The javax.xml.bind.* API was removed from the standard library in
newer Java versions. Hence we have to declare an explicity dependency
on it, for Java 11 or higher.
