Add a "title" parameter to the TOC macro

Adding a title to a table of contents (TOC) currently requires wrapping the TOC in a Box like on this example. The new title parameter will allow to insert a heading in the TOC macro  directly.

The TOC macro should then use the provided heading as an aria-label or reference the heading using aria-labelledby, with a fallback to a predefined value of the aria-label attribute when the heading is empty, as proposed by MichaelHamann in this discussion.

The TOC heading id should be unique since there can be several TOCs in a page, eg using the XDOM id generator.