Data source activated and changed signals

[cryos]

PR #1709 emits data source changed when a data source is activated. I think short term we can merge, and examine whether this fixes the immediate bug. I want to keep this issue open so that we revisit whether two separate signals are feasible with the updated pipeline. Ideally they should be, and we can improve the update logic.

dataSourceChanged: indicate that the data source has changed, and deeper updates might be necessary to account for the change.

dataSourceActivated: indicate that the active data source changed, intended to be a smaller change requiring less updates.

If there is no utility in keeping them separate we should look at removing dataSourceActivated, and only using dataSourceChanged. It would be preferable to keep both if we can be reasonably clear they have different but related purposes.