Improved the returned count of cancelled workflows

[raymonddenhaan]

• Removed dispatched child workflows from the count
• Removed finished workflows from the count
• Prevented dispatching a cancellation request for finished workflows as they can not be cancelled

It is still not 100% accurate, however it is less confusing to the user.
Not 100% since we can not guarantee that all dispatched cancellation requests also lead to a cancelled workflow. (Could be finished by the time we actually try to cancel it)

[sfmskywalker]

Perhaps we should consider adding a "Canceling" status to workflow instances. This way, we could immediately set the status of the selected workflow instances to Canceling - which would be reflected by the UI.
The UI could then potentially receive realtime updates via SignalR (or simple polling) as workflow instances become canceled.

To be clear: this would be out of scope for this PR 😄