Issue for Tal's PR 14675, dependency of #33610.

1. Only create CodeContext instances for "real" editors windows, but not e.g. shell or output windows.

- Previously, were created but never activated because menu item deactivated.  This is even better.

2. Remove configuration update Tk event fired every second, by having the editor window ask its code context widget to update when necessary, i.e. upon font or highlighting updates.

3. When code context isn't being shown, avoid having a Tk event fired every 100ms to check whether the code context needs to be updated.

4. Use the editor window's getlineno() method where applicable.

5. Fix a bare except:.