[Divina] Have Link Object be the default format (rather than a URI) each time a file is expected

[anymus]

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At this stage, the transitions module lists 2 Transition Object keys whose values can contain URIs: file (for a video transition) and sequence (for a sequence transition) - file is a URI while sequence is an array of URIs.

To be consistent with the rest of the specifications, and since providing additional information like alternate resources or a fit value could be useful, I'd recommend changing the expected format to Link Object for file, and array of Link Objects for sequence.

(Note that the alternate array is itself an array of Link Objects)