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.
diff --git a/Lib/idlelib/config-main.def b/Lib/idlelib/config-main.def
index b2be625..28ae941 100644
--- a/Lib/idlelib/config-main.def
+++ b/Lib/idlelib/config-main.def
@@ -59,6 +59,7 @@
 [EditorWindow]
 width= 80
 height= 40
+cursor-blink= 1
 font= TkFixedFont
 # For TkFixedFont, the actual size and boldness are obtained from tk
 # and override 10 and 0.  See idlelib.config.IdleConf.GetFont