Refactor queueing system to allow auto-dequeuing to use dequeue directly and

Refactor queueing system to allow auto-dequeuing to use dequeue directly and
therefore require dequeue to always leave the element it has dequeued on
the queue.

  - In the fx queue, a progress sentinel is added when a queue element
    is dequeued.
  - The sentinel is ignored when dequeue is called explicitly
  - When adding a new element to the fx queue, queue() checks if
    the progress sentinel is present. If not, it calls dequeue()