gh-59396: Use themed widgets in tkinter.filedialog (GH-152036)

gh-59396: Use themed widgets in tkinter.filedialog (GH-152036)

The FileDialog, LoadFileDialog and SaveFileDialog dialogs are now built from
the themed tkinter.ttk widgets by default instead of the classic tkinter
widgets, and gained a use_ttk parameter that selects between the classic Tk
widgets and the themed ttk widgets.

They were also brought closer to the native Tk file dialog: the buttons and
field labels gained Alt key accelerators, the default ring follows the keyboard
focus, the Escape key cancels the dialog, the focus traverses the widgets in
their visual order, and the directory and file lists gained a horizontal
scrollbar and type-ahead selection.  The dialog is now transient and centered
over its parent, and the SaveFileDialog overwrite confirmation uses a themed
message box.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>