GUI crashes with freePtr when stepping through code on Windows 8

When ~~playing with~~ testing bugjar on Windows 8 I discovered that the UI component will consistenly crash with the following error on the console.

```
TclStackFree: incorrect freePtr. Call out of sequence?

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
```

Based on this [link](http://grokbase.com/t/python/tkinter-discuss/112922445t/tclstackfree-error), it seems that this error is caused by more than one thread accessing the tk event loop.

My hypothesis is that the Debugger is invoking callbacks such as MainWindow.on_stack and MainWindow.on_line from the non tk event thread.

Below is an attempt to rectify this situation using the python Queue object.

``` python
class MainWindow(object):
def __init__(self, root, debugger):

# ------------

self.queue = Queue.Queue()
self.root.after(200, self.debugger_update)

debugger.start()

def debugger_update(self):
try:
while 1:
func_name, args = self.queue.get_nowait()
handler = getattr(self, func_name)
handler(*args)
except Queue.Empty:
pass
self.root.after(100, self.debugger_update)

# ------------

def on_stack(self, stack):
self.queue.put(('_on_stack', (stack,),))

def _on_stack(self, stack):
# ------------
```

Using the above setup, I have found that I am able to step through the code without encountering the same freePtr issue.