Graph grouping: display cycle point of grouped families

[MetRonnie]

The cycle point should be displayed in the label when grouping by family (otherwise you can end up with multiple identical groups).

Moved from #1130

[oliver-sanders]

The original issue suggested using the whole GraphNode for the label but making it translucent. This way we can operate on the whole group (click on group task icon).

This may need workshopping a touch, especially after the decision to use "opacity" to indicate no-flow tasks.

[MetRonnie]