Allow "Use as activity" activities to dispatch/execute the workflow

Currently, we can use the workflow designer to create a workflow and configure it to be usable as an activity.
This will make the workflow available as an activity from the activity picker, and act as a composite activity that executes within the consuming workflow instance.

What we could do as well, is allow the user to actually execute the workflow as a separate workflow instance.
This way, the user doesn't necessarily have to use the Dispatch Workflow activity, which doesn't display any of the user-defined input/output/outcomes of the workflow definition activity.