bpo-42721: Improve using simple dialogs without root window (GH-23897)

When simple query dialogs (tkinter.simpledialog), message boxes
(tkinter.messagebox) or color choose dialog (tkinter.colorchooser)
are created without arguments master and parent, and the default
root window is not yet created, a new temporary hidden root window
will be created automatically. It will not be set as the default root
window and will be destroyed right after closing the dialog window.
It will help to use these simple dialog windows in programs which do
not need other GUI.

Previously, message boxes and color chooser created the blank root
window and left it after closing the dialog window, and query dialogs
just raised an exception.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/tkinter/__init__.py

@@ -300,6 +300,31 @@ def _get_default_root(what=None):
     return _default_root
 
 
+def _get_temp_root():
+    global _support_default_root
+    if not _support_default_root:
+        raise RuntimeError("No master specified and tkinter is "
+                           "configured to not support default root")
+    root = _default_root
+    if root is None:
+        assert _support_default_root
+        _support_default_root = False
+        root = Tk()
+        _support_default_root = True
+        assert _default_root is None
+        root.withdraw()
+        root._temporary = True
+    return root
+
+
+def _destroy_temp_root(master):
+    if getattr(master, '_temporary', False):
+        try:
+            master.destroy()
+        except TclError:
+            pass
+
+
 def _tkerror(err):
     """Internal function."""
     pass