[Workflow] Being able to have more than one initial_place when using multiple_state store

[Korbeil]

Description
When we are using multiple_state marking store, we should be able to have multiple initial_place as well.

Example
Before

framework:
  workflows:
    order:
      supports:
      - 'Foo\Bar\Order'
      audit_trail:
        enabled: true
      type: workflow
      marking_store:
        type: 'multiple_state'
        arguments:
        - 'marking'
      places:
      initial_place: 'order_created'

After

framework:
  workflows:
    order:
      supports:
      - 'Foo\Bar\Order'
      audit_trail:
        enabled: true
      type: workflow
      marking_store:
        type: 'multiple_state'
        arguments:
        - 'marking'
      places:
      initial_place:
      - 'order_created'
      - 'order_cart'

Since multiple_state allow us to handle more than one marking, it should be same for initial_place.

[Nyholm]

👍
Great suggestion. I think this makes a lot of sense.