bpo-29446: tkinter 'import *' only imports what it should (GH-14864)

Add __all__ to tkinter.__init__ and submodules.  Replace 'import *'
with explicit imports in some submodules.
diff --git a/Lib/tkinter/dialog.py b/Lib/tkinter/dialog.py
index cb463f7..8ae2140 100644
--- a/Lib/tkinter/dialog.py
+++ b/Lib/tkinter/dialog.py
@@ -1,7 +1,8 @@
 # dialog.py -- Tkinter interface to the tk_dialog script.
 
-from tkinter import *
-from tkinter import _cnfmerge
+from tkinter import _cnfmerge, Widget, TclError, Button, Pack
+
+__all__ = ["Dialog"]
 
 DIALOG_ICON = 'questhead'