Add getOutermostTypeElement to javax.lang.model utility class

This is a useful utility used in the doclet's new api and would be beneficial to other users of javax.lang.model.

Suggested code:
    public TypeElement getOuterMostTypeElement(Element e) {
        if (e.getKind() == ElementKind.PACKAGE)
            return null;
        Element encl = e.getEnclosingElement();
        ElementKind kind = encl.getKind();
        if (kind == ElementKind.PACKAGE)
            return null;
        while (!(kind.isClass() || kind.isInterface())) {
            encl = encl.getEnclosingElement();
        }
        return (TypeElement)encl;
    }