100% CPU after reading is aborted

[Lithopsian]

Press ctrl-S during a tree read to abort the read. CPU usage goes to 100%. This may not be noticeable at first as the gui is not blocked, but some actions such as opening a new dialog may become unresponsive.

The directory read queue runs off a zero-length timer. When a read is aborted, the queue is emptied but the timer is not stopped. It now fires continually with no other activity to slow it down.

A simple fix is to stop the timer when the queues are cleared. A more robust fix seems to be to stop the timer whenever the queue is found to be empty, although I haven't found any cases other than the abort that run out of control. I'll submit a pull request for that approach.

[shundhammer]

This should now be fixed with #257.