epub: generate a title toc-entry event

The previous code used the title toc-entry event from the NCX or
HTML5 toc/nav document. This had several problems:

  1.  The toc-entry is only present for NCX documents that have a
      docTitle element.

  2.  The title of the event is the one from the toc document, not
      the ePub document (dc:title/dct:title in the OPF file).

  3.  The anchor points to the toc document, not the ePub document.

      This is a problem for the HTML5-based toc documents that can
      have the toc document anywhere in the spine, so the title
      toc entry could link to content anywhere in the document.

To resolve these issues, the following changes have been made:

  1.  Ignore the title toc-entry event from the toc document.

  2.  Generate a toc-entry at the start of the epub event stream
      using the title in the OPF file.