Issue #16893: Replace help.txt with idle.html for Idle doc display.

The new idlelib/idle.html is copied from Doc/build/html/idle.html.
It looks better than help.txt and will better document Idle as released.
The tkinter html viewer that works for this file was written by Rose Roseman.
The new code is in idlelib/help.py, a new file for help menu classes.
The now unused EditorWindow.HelpDialog class and helt.txt file are deprecated.

Lib/idlelib/EditorWindow.py

@@ -21,6 +21,7 @@ from idlelib import PyParse
 from idlelib.configHandler import idleConf
 from idlelib import aboutDialog, textView, configDialog
 from idlelib import macosxSupport
+from idlelib import help
 
 # The default tab setting for a Text widget, in average-width characters.
 TK_TABWIDTH_DEFAULT = 8
@@ -42,6 +43,11 @@ def _sphinx_version():
 class HelpDialog(object):
 
     def __init__(self):
+        import warnings as w
+        w.warn("EditorWindow.HelpDialog is no longer used by Idle.\n"
+               "It will be removed in 3.6 or later.\n"
+               "It has been replaced by private help.HelpWindow\n",
+               DeprecationWarning, stacklevel=2)
         self.parent = None      # parent of help window
         self.dlg = None         # the help window iteself
 
@@ -539,11 +545,13 @@ class EditorWindow(object):
         configDialog.ConfigExtensionsDialog(self.top)
 
     def help_dialog(self, event=None):
+        "Handle help doc event."
+        # edit maxosxSupport.overrideRootMenu.help_dialog to match
         if self.root:
             parent = self.root
         else:
             parent = self.top
-        helpDialog.display(parent, near=self.top)
+        help.show_idlehelp(parent)
 
     def python_docs(self, event=None):
         if sys.platform[:3] == 'win':
@@ -1716,4 +1724,4 @@ def _editor_window(parent):  # htest #
 
 if __name__ == '__main__':
     from idlelib.idle_test.htest import run
-    run(_help_dialog, _editor_window)
+    run(_editor_window)