filtering: consider task modifier filtering

[oliver-sanders]

We can filter by task states, but not task state modifiers.

It might be useful in some cases to filter for modifiers, main use cases:

• Filtering out runahead tasks (these are superfluous for most use cases and were filtered out by default at Cylc 7).
• Filtering for queued tasks (e.g. I want to see all "active" tasks in a workflow i.e. waiting(queued), preparing, submitted, running where this definition of "active" differs subtly from the n=0 definition which also contains tasks with partially satisfied prereqs).

See also the queue view.