Used to convert a t-field specification into an output HTML field.
:meth:`~.to_html` is the entry point of this conversion from QWeb, it:
* converts the record value to html using :meth:`~.record_to_html`
* generates the metadata attributes (``data-oe-``) to set on the root
result node
* generates the root result node itself through :meth:`~.render_element`