Related docs: able to manually select folders as related document

Looks like we could use a check 

if (selectedNodeModel.getNode().isNodeType(HippoStdNodeType.NT_FOLDER)) {
    error("You cannot add a folder as the related document");
    return;
}

in org.onehippo.forge.relateddocs.editor.DocumentPickerDialog

Moreover, if a folder is selected and the document closed and re-opened, there's a ClearDeletedDocumentsDialog saying "One or more related document references have been cleaned up because the referenced document no longer exists."

Also see HIPPLUG-1600 for localization support.