Take a quick stroll through the unchartered corners of the DOM node data
structures:
- Remove ununsed struct dom_node_id_item.
- Make the document node reference a future struct dom_document.
- Describe ideas for node data, e.g. the entity reference node should use
it for storing the unicode_val_T.
This changes init_dom_node_() to take an allocated argument saying whether
to allocate or not. If the value is -1, node->allocated will be set to the
value of node->parent->allocated. This way the value is inherited like we
do it in the menu code. It should be a sane default since we eventually
want not to rely on the 'underlying' source of the document and there will
be less variables to pass around.
