bpo-4630: Add cursor no-blink option for IDLE (GH-16960)

This immediately toggles shell, editor, and output windows, but does not affect other input widgets.

Lib/idlelib/config.py

@@ -158,6 +158,8 @@ class IdleConf:
         self.defaultCfg = {}
         self.userCfg = {}
         self.cfg = {}  # TODO use to select userCfg vs defaultCfg
+        # self.blink_off_time = <first editor text>['insertofftime']
+        # See https:/bugs.python.org/issue4630, msg356516.
 
         if not _utest:
             self.CreateConfigHandlers()