Popup footnotes in EPUB 3

Hello, 

Please read https://github.com/edrlab/thorium-reader/issues/2833 and Daniel's comments about the lack of specification of popup footnotes. 
I found definitions for "footnote" and "noteref" in the EPUB 3.3 spec, but nothing related to popup footnotes in the RS spec.
The upcoming work on EPUB 3.4 seems like a good time to properly specify popup footnotes. 

Best regards,
Laurent Le Meur | EDRLab