Add audio and audio mixing

[nigelmegitt]

Closes #105 and #71.

• Add Audio, Synthesized Audio, Audio Recording and Mixing Instruction to the data model
• Add informative section explaining audio mixing, with figures
• Add extensions and apply constraints to content and processor profiles to support the required audio features
• Reference "question" issues about the ways to reference or embed audio resources
• Create the concept of "shared properties" and a subset of those that are "timing properties", so that Begin, End and Duration can be defined once and reused in all the classes where they apply
• Fix some linting issues
• Explain how extended descriptions can be implemented but are not specified
• Define handling of multiple <source> element children of <audio>

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Preview | Diff

[cconcolato]

My overall comment on the PR is not about the editing (I have minor comments/questions) but more about the complexity of the solution. We are leaving too many options: src vs source, fragment identifiers, external files vs embedded, base64 vs others, audio codecs, multiple sources .... I would prefer if we went for a simple solution first and then add more features.

In terms of editing, some comments:

• can we integrate section 5 inside 4.10 or move it as an Annex?
• the spec is starting to get big (47 pages!) The diagrams are taking a lot of space (and some of the open issues). I'm wondering if we should move the large diagrams to annexes.

[nigelmegitt]

We are leaving too many options: src vs source, fragment identifiers, external files vs embedded, base64 vs others, audio codecs, multiple sources .... I would prefer if we went for a simple solution first and then add more features.

I agree - the difficulty I have is in knowing which of the options to retain and which to omit. I want industry feedback so I have raised question issues about them. Seems to me that the best idea is to see what people want and need and remove everything else.

[nigelmegitt]

• can we integrate section 5 inside 4.10 or move it as an Annex?

I added an editorial note asking something similar! Done in f2d0194.

[nigelmegitt]

• the spec is starting to get big (47 pages!) The diagrams are taking a lot of space (and some of the open issues). I'm wondering if we should move the large diagrams to annexes.

In my view, that would not be an improvement. By closing open questions issues we will save some space, for sure. I'm not even sure that the length is a problem per se - I think we're targeting readability and conceptual explanations over length considerations.

I just did a print to pdf preview and on my machine at A4 size it comes to 44 pages, of which 23 are the main body - data model, constraints and conformance, and they start on page 8. Everything from page 32 onwards is appendix.