Support footnotes

[bertfrees]

To do:

• footnotes page area and max-height property
  • [x] css-utils (Java)
  • [x] css-utils (XML)
  • [x] dotify-formatter
• ::footnote-call pseudo-element
  • [x] css-utils (XML)
• [x] Flow footnotes to the bottom of the page on which the footnote reference occurs (::alternate + target-content())
• [ ] Automatic numbering of footnotes
• [ ] footnote-policy

Depends on:

• [ ] https://github.com/snaekobbi/braille-css-spec/issues/3
• [ ] https://github.com/snaekobbi/braille-css-spec/issues/6
• [x] https://github.com/snaekobbi/pipeline-mod-braille/pull/7
• [x] https://github.com/snaekobbi/pipeline-mod-braille/issues/12
• [ ] https://github.com/joeha480/dotify/issues/100

[bertfrees]

Started working on branch footnotes_new (based on flow_new)