BeanProvider public method requires access to a package-private interface

I mentioned this in http://jira.codehaus.org/browse/XSTR-601, but figured it's a separate bug and needs to be tracked separately.

BeanProvider has a public method:

public void visitSerializableProperties(Object object, Visitor visitor)

that, even though it's public, can't be used outside of the package com.thoughtworks.xstream.converters.javabean due to Visitor being a package-private interface.

This makes it close to impossible to override methods such as JavaBeanConverter.marshall() without completely re-writing them from scratch.