Use QueueTaskDispatcher canRun() instead of canTake(), add status of que...

...uing and button for dequeue

- Jenkins obviously tries quite often to start the queued builds, if the call to canTake() returns null. canRun() makes it to happen not so often.

- It's possible that sometimes a queued resource will stay queued forever. Now this status can be seen and manipulated in the lockable resources view.