Update bean writer support on behalf of Related Docs providing a bean

Now related documents plugin has been extended by a compound bean class in a feature branch.
[see https://svn.onehippo.org/repos/hippo/hippo-cms7/plugins/relateddocs/branches/hippo-plugin-relateddocs-add-hst-3.1.xx]
Please create another feature branch for essentials and make sure essentials works with updated related documents plugin.