Renamed break exception to fit convention.
Commented break tests.
Reorder of methods for easier comparison with Break class.
Removed duplication of state checks.
Slowed down the tests to make them pass (eeek!).
Fixed the UI to deal with the fact Tkinter is not thread safe. By
convention, all UI functions must be in the same thread as the root
frame.
The solution is for writer threads to write to a queue which is serviced
only by the main thread.
Renamed method and commented.