Adding blank data for non-smilled spine items

[yuriyp]

No description provided.

[danielweck]

Thank you, I will be using your code, but I will probably move it to SmilDocumentParser.js, as I am fixing a more critical parsing bug there.

See my related message, sent earlier today:
"
I was running tests against the MO EPUB file 120 from
epub-testsuite ( https://github.com/mgylling/epub-testsuite/tree/master/content/30/epub30-test-0120 ), and a specific fail caught my attention. I looked at
the parsing code ( https://github.com/readium/readium-js/blob/develop/epub-modules/epub/src/models/smil_document_parser.js#L134 ), and realised that the generated sequence of SMIL
data is incorrectly based on the order of manifest items (instead of
the spine).
"

Daniel