Add helpers to QueueMonitor.

It now has addOperationToQueue and addOperationToQueueWithBlock,
and addOperation is renamed to monitorOperation for clarity.

These can add the operation to the queue and monitor it in one step.
Add an isNetwork parameter to the initializer.  This can be used by
callers to indicate whether the network activity indicator should be
shown for this queue.

Also, change the activity indicator so that it waits 0.3 seconds before
changing state.  This avoids jitter when lots of short items are
being added to the queue.