Promote supported properties as part of DataViewModel API

[connor15mcc]

After re-examining the DataViewModel API, we've made explicit the support for groupSortFn, showGroupRowCounts, and hideEmptyTextBeforeLoad. Our previous "escape hatch" to pass parameters to GridModel did not call attention to the lack of formal support for these properties -- recent changes (#3421) have highlighted this and motivated this corresponding change.

Associated toolbox PR: xh/toolbox#662

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• Caught up with develop branch as of last change.
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