[3.15] gh-88758: Handle non-tkinter widgets in tkinter focus methods (GH-152337)...

[3.15] gh-88758: Handle non-tkinter widgets in tkinter focus methods (GH-152337) (GH-152346)

focus_get(), focus_displayof(), focus_lastfor() and winfo_containing()
now return None instead of raising KeyError when the focused widget was
not created by tkinter (for example a torn-off menu).
(cherry picked from commit 5fed5ce85d9c862673cc68294f757f345bbcc9b1)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>