WorkflowLayout.start makes it difficult to update ViewEnvironment on the fly

[rjrjr]

WorkflowLayout.start takes a ViewEnvironment argument, and offers no way to update it after the first rendering. This can be worked around by judicious use of DecorativeViewFactory, but that's a pain and I suspect this will be a pretty common use case.

I think we should change the signature thus:

fun start(renderings: Flow<WorkflowRoot<Any>>)

class WorkflowRoot<R>(
  val rootRendering: R,
  val viewEnvironment: ViewEnvironment = ViewEnvironment()
)

[rjrjr]

Fixed by EnvironmentScreen and API updates in #591.