ghost tasks: switch from flow=None to graphDepth=0

[oliver-sanders]

We currently display flow=None tasks in grey (ghost tasks in Cylc 7 slang).

Status Quo

The motivation for this was:
a. Indicate cylc removed tasks.
b. Visually distinguish between n=0 and n>0 waiting tasks.

As a side effect, any task actually triggered in flow=None (extremely rare) would also show in grey.

Issues

There are some issues with this approach:

1. n>0 waiting tasks are grey, but n<0 finished tasks are not
   • This give us an asymmetry in the time dimension which is perhaps confusing.
   • Sometimes it's useful to differentiate between n=0 and n<0 finished tasks, e.g, failed tasks where success is optional.
2. Tasks get greyed out, but families don't:
   • Families don't have flow numbers because they are groupings.
   • If the tree view is collapsed down, there is no visual indication of a removed family.
   • This also gets confusing with group-trigger.
3. Manually triggered --flow=None tasks get greyed out:
   • Arguably either confusing or a feature depending on standpoint.
   • Extremely rare, so not a major concern for us.

Alternative

A, perhaps, more logical approach would be to grey out n!=0 tasks.

This would still answer to the original use cases:
a. cylc removed tasks are all n!=0 by definition.
b. n>0 tasks are obviously n!=0!

This would wipe out the issues above.