Clear associated breakpoints when closing an edit window.

Clear associated breakpoints when closing an edit window.

M Debugger.py      : Added clear_file_breaks()
M EditorWindow.py  : Clear breaks when closed, commments->docstrings,
                     comment out some debugging print statements
M PyShell.py       : comments->docstrings ; clarify extending EditorWindow
                     methods.
M RemoteDebugger.py: Add clear_all_file_breaks() functionality,
                     clarify some comments.