DOC: TOC update

* Check the whole style inheritance tree to decide whether the current
paragraph is a heading.
* Added support for the \t switch of the TOC field. If this one is present,
then TOC includes paragraphs formatted with styles other than the built-in
heading styles. Making use of text:index-source-styles elements.
* Provide the style-name in the text:index-title-template element if possible.