Add methods to load / refresh a TabContainer directly

[TomTirapani]

See issue: #2406

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[lbwexler]

Don't fully understand this change, and would like to discuss, but it does seem to me that you should only need to override loadAsync and everything else should follow.

[lbwexler]

@TomTirapani -- Looks appealingly simple! Looking forward to discussing. Am a little concerned about unexpected effect of adding so many new RefreshContext's in, but it makes a lot of sense.

[TomTirapani]

I just came across another use-case for this in a client app (I was able to work around it)

[amcclain]

Tried (and failed) to get the bundled yarn to stop showing up here as a diff, but I don't think anything actually changed. (Some line ending thing, maybe?)

[TomTirapani]

I fixed it with git checkout 2e9b578a4db1a4e9a7c1a3f75cd6a0101234cb91 -- .yarn/releases/yarn-1.22.19.cjs

[lbwexler]

I still have a lingering question about whether we should have TabContainerModel delegate its doLoadAsync to this model. Seems like as an API user I would expect to just call refresh methods directly on the TabContainerModel, but am concerned we are somehow going to get double loading.

[lbwexler]

tom reminded me that we discussed this at length in the spring, and we actually decided not to do that delegation, after originally trying it.

We agreed to merge this as is so we can get the benefits of the new refreshContextModel, and shake out any issues. Can put a comment in the code, about potentially taking it further with a delegation.