Message 224062 - Python tracker

This refactoring is required to enable setting/clearing breakpoints, using linenumbering( whether using a Canvas implementation or a Text implementation http://bugs.python.org/issue17535)
The patch ensures consistency between the set_breakpoint(_here) and
clear_breakpoint(_here) methods. The clear_breakpoint_here, which is not present currently has been added.
No functionality has been added or removed, so I think it should be safe to go forward.