Message 144809 - Python tracker

This issue is very similar to issue5707: it is possible to define a custom key binding to "<Alt>" or "<Control>": just click the Alt box and don't select a letter.
There is no check, it's possible to save this buggy key binding, and IDLE won't start anymore.

IDLE should:
- check the validity of the binding and refuse to save it when it is invalid
- gracefully skip invalid bindings from the config file