Extend Range Selector, add Multi Selector

[iherman]

@tcole3 @BigBlueHat,

I had some empty time today, so I created this PR. It is based on my thoughts on what we had so far, and I believe that this design covers the use cases, namely:

• Range that spreads over several resources (by extending the Range Selector)
• Selection of a series of other, disjoint selections (cf. the example of @RachelComerford).

It also seems to be semantically clean, avoiding some issues in my comments.

It required quite a number of changes on the document (see the list of changes below) so, I'm afraid, the diff file does not really make sense. But the preview might be fine.

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Preview | Diff

[iherman]

Here is the list of changes:

• Added the extension to the Range Selector, with the relevant semantics
• Renamed the Multi Selector
• Updated all the examples
• Moved the Refinement section before the range and the multi selector to ensure proper cross references
• Documented the changes in the introduction and the appendices
• Added a note (in the refinement selection) referring to the fact that 'scope' can, in some cases, express the same information more succinctly (ie, it is worth keeping it after all).
• Added/updated the fragment identifiers
• Reformatted the examples, as well as the fragment examples to reduce indentation (and, thereby, avoiding horizontal scrolling)
• The refinement had to be extended with a reference to Positions as possible values.

(The last two items are not really dependent on this PR, but I did it on the fly; if we decide to reject this PR, that may have to be repeated on the master branch...)

[tcole3]

The PR will allow us to discuss several issues, and so I would like to see it merged. The issues for me are:

• Is the modeling of the new Embedded Resource Selector consistent with (i.e., a reasonable additional type of) the Selector class as defined by Web Annotations? I would argue it is. In example 10 you have a resource (MobyDick.wpub, a Web Publication) and you want to identify and reference a segment of it that happens to have it's own URL (moby-dick-book-cover.jpg). To me the fact that the segment has its own URL does not invalidate using a Selector to reference it, but this is arguable and needs to be discussed.

• A second question is whether we need to redefine Range Selector? I would argue that we do not, at least we do not if we have a multi selector, since the use cases addressed by extending the Range Selector look to me like a subset of the use cases that can be expressed using the multi selector. Again this warrants discussion.

• A third question is whether multi selector is in fact a Selector (as with first question)? Elsewhere @azaroth42 has said, "I think multiple selections should be at the SpecificResource [locator] level, the same way as we can have multiple targets in an Annotation." I would suggest that this is dependent on the answer to question 1. If ERS is a valid Selector, then the multi Selector seems okay. But if ERS is not a valid type of Selector, then multi Selector is also not a Selector, and we would need to fall back on the multi-locator approach previously proposed.

Anyway, let's merge this PR now to facilitate discussion of these issues Tuesday afternoon.