Add "Group snapshot: Take/Load" targets #394
Description
This would be an alternative to SWS snapshots. We should have multiple snapshot slots. Offer a target for taking a snapshot and saving it in a slot (of all snapshottable targets in the same group). And a target for loading a snapshot in a slot.
Loading a snapshot is in most cases not more than hitting the target with the absolute control value saved as part of the group snapshot. However, targets like "FX: Load snapshot" should provide special behavior, e.g. returning the chunk of the currently loaded FX.