Rework SimpleDialog and Dialog to match the look and feel of the native Tk
dialogs ::tk_dialog and ::tk::MessageBox.

* SimpleDialog is a Python port of ::tk_dialog and Dialog a base class
  modelled on ::tk::MessageBox.  Both adopt the message-box keyboard
  conventions: button accelerators, a default ring that follows the keyboard
  focus, and a <Return> binding that invokes the focused button.
* Both classes gain a use_ttk parameter that selects the classic Tk or the
  themed ttk widgets.  It controls the widget set and the appearance that the
  two procedures style differently, but not the keyboard behaviour.
* Update _place_window with the Tk 9.1 placement refinements.
* The new helpers _temp_grab_focus (a modal grab/focus context manager),
  _underline_ampersand and _find_alt_key_target (ports of the Tk
  accelerator-key procedures) can be reused by other tkinter dialogs, as
  _setup_dialog already is.
* Fix several defects uncovered while comparing with the Tcl sources.
* Improve the simpledialog demo.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

… holds the grab (GH-157677)

_temp_grab_focus() used grab_current() and focus_get(), which raise
KeyError for windows which were not created by tkinter, such as a
native message box.  Use Tcl window names instead of widgets.

Regression from GH-151848, not present in released versions.