Fix _SelectionModel and _MultipleSelectionModel to sync using indices

[jasongrout]

Fixes #1091

• Fix javascript side
• Close Possibility to get key of selected items in Select(Multiple) #880 when this is merged.

CC @maartenbreddels

[maartenbreddels]

If we sync label too, that would be useful in some cases, what do you think?

[jasongrout]

@maartenbreddels, can you give this a try and see how it works?

If we sync label too, that would be useful in some cases, what do you think?

Possibly, though I don't like having two possibly conflicting sources of truth in the protocol, and it becomes confusing on who sets it (frontend or backend? Both I think have a valid usecase).

[jasongrout]

If we sync label too, that would be useful in some cases, what do you think?

I'd much rather have the label derived from the index independently on the frontend and backend.

[jasongrout]

Two design decisions:

(a) in this implementation, the default selection is no selection (except for the selection slider, which must have something selected

(b) We always coerce the options list to a ((label, value), (label, value)...) tuple. I think before it kept whatever object was originally assigned. I like this better - it emphasizes the ordered aspect of the options. However, for convenience, we still allow (and even expand) possible input types to include any mapping or iterable.

Thoughts?

[jasongrout]

(a) in this implementation, the default selection is no selection (except for the selection slider, which must have something selected

I went ahead and changed this to always select the first item (if there is a first item) on initialization or when changing the options. That seems to work better with interact, for example, where you just want some value by default.

However, I left the multiple select to still unselect all items when changing the options or setting it initially. Thoughts?

[jasongrout]

However, I left the multiple select to still unselect all items when changing the options or setting it initially. Thoughts?

(This is a change - before, if the options changed in a multiple select, it would try to match up the previous selected labels to attempt to select the same items after the change. I think that is very error-prone.)

[jasongrout]

Merging...let's iterate in another PR.