bpo-37530: simplify, optimize and clean up IDLE code context (GH-14675)

bpo-37530: simplify, optimize and clean up IDLE code context (GH-14675)

* Only create CodeContext instances for "real" editors windows, but
  not e.g. shell or output windows.
* 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.
* 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.
* Use the editor window's getlineno() method where applicable.
* Update font of the code context widget before the main text widget
(cherry picked from commit 7036e1de3a87d36c7ef41b8a2b44ed6fc4d34be2)

Co-authored-by: Tal Einat <taleinat@gmail.com>