Add a 'description' field to the schema for states and transitions

This would be very helpful, I needed to add a custom transition to our commerce orders so shipping emails didn't get sent and it would be very helpful to have some description text to remind the users which transition sends the emails and which doesn't.

Two issues I can see:

1. Constructor signature changes for WorkflowState and WorkflowTransition.
2. I'm not sure whether/how to surface the state description anywhere, OrderListBuilder doesn't really seem to be used since that's normally a view so there would have to be something done to add a title attribute to that cell.

I have tested the feature: very useful for my situation.

I have read the diff (code changes): looks good to me.