When attempting to open a file that doesnt exist or has permission
problems, the application would end after hitting OK on the message
box instead of starting up to an empty editor.
I believe this has something to do with the dialog event loop
interfering with the application event loop, or possibly the fact
that the window creation code is in the show() method.
To work around this I now call the open_file() method after the
show() method.
If you had made a change to the first 4 bits of a byte and then changed
your offset via keyboard or menu option it would change the last 4 bits
of the new offset the next time you made a change and would not show that
the byte had been changed at the new offset.
