PEF preview with real links in the TOC

[bertfrees]

This would be a huge improvement of the user experience of navigating a PEF document.

With some recent changes in OBFL and Dotify it has become technically feasible to implement this. The idea is the following: we insert special tokens in the OBFL before and after <page-number ref-id="xxx"/> and a corresponding token before <span id="xxx"/>. These tokens will flow through the processor and end up in the PEF at the expected locations. pef:store then interprets this metadata to generate links in the HTML.