bpo-30928: IDLE - update NEWS.txt. (GH-5539) (GH-5540)

Add entry for uniform lower context; add 'since' to be explicit. 
(cherry picked from commit 05e806767b857b1eab838e712828e3a7d57cabf1)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
diff --git a/Lib/idlelib/NEWS.txt b/Lib/idlelib/NEWS.txt
index 9784c46..d616a74 100644
--- a/Lib/idlelib/NEWS.txt
+++ b/Lib/idlelib/NEWS.txt
@@ -1,8 +1,11 @@
-What's New in IDLE 3.7.0
+What's New in IDLE 3.7.0 (since 3.6.0)
 Released on 2018-06-18?
-========================
+======================================
 
 
+bpo-32765: Update configdialog General tab create page docstring.
+Add new widgets to the widget list.
+
 bpo-32207: Improve tk event exception tracebacks in IDLE.
 When tk event handling is driven by IDLE's run loop, a confusing
 and distracting queue.EMPTY traceback context is no longer added
@@ -294,9 +297,9 @@
 Update and augment description of the configuration system.
 
 
-What's New in IDLE 3.6.0
+What's New in IDLE 3.6.0 (since 3.5.0)
 Released on 2016-12-23
-========================
+======================================
 
 - Issue #15308: Add 'interrupt execution' (^C) to Shell menu.
   Patch by Roger Serwy, updated by Bayard Randel.