gscan: omit stopped workflows from cumulative state totals

[oliver-sanders]

The cumulative state totals include the states of all workflows in the hierarchy.

The Cylc UI doesn't list tasks for stopped workflows, unless the workflow stopped whilst the UI Server was connected to it, in which case it preserves the last known state of the workflow before it shut down.

This can lead to the slightly confusing situation where a stopped workflow is reported as having active tasks. These tasks contribute the gscan state totals (even if stopped workflows are filtered out) which isn't great.