Add missing in-line documentation to ScrollingListPopulator;
noticed in the context of work done for WTD-599,
refactoring of time- and value-formatting for telemetry
into a reusable, injectable service.
Fill in specs for ScrollingListController, and for
the specific Column types that support it. Separate
out the code that produces actual rows in order to
improve testability and maintainability. WTD-534.
