first version of a SysReadSelectionHandle (manual sweep of !53339)

The idea is to allow reading systematics varied selection decorations with the existing ISelectionAccessor infrastructure and being fully integrated with our systematics handles.

I updated all the algorithms to use this, but haven't yet updated the configuration to use it, and also haven't added any specific tests for this purpose.

The automatic sweep failed due to some trivial differences, which I resolved manually. Please review this as if it was a regular sweep.