gh-86726: Document the full public API of tkinter

[serhiy-storchaka]

Replace the previously sparse reference documentation with full coverage of the public API of the tkinter package, written from the Tcl/Tk manual pages, the existing documentation and the module docstrings.

• Doc/library/tkinter.rst gains a "Reference" section documenting every public class, method, function and constant of the core module -- the widgets, the Misc, Wm, Pack, Place, Grid, XView and YView mix-ins, the Variable and image classes, the module-level functions and the symbolic constants.
• Doc/library/tkinter.ttk.rst, dialog.rst, tkinter.font.rst and the other module pages document their remaining classes, methods and functions.

The descriptions are Python-oriented (correct return types -- tuples rather than Tcl lists, booleans, integers, None on cancellation, and so on) and were checked against the Tcl/Tk 9.1 manual pages and the implementation.

versionadded, versionchanged and deprecated directives are added for the public API, determined from the git history relative to Python 3.0: the tkinter.ttk module (3.1); the Text, Wm, Menu and Misc methods exposing Tk 8.5 features (3.3); and the many later additions and behavior changes up to 3.15. The Tk version required by features added after Tk 8.6 is noted as well. The bundled Tcl/Tk version is updated to 9.0 and the manual-page links point at the tcl9.0 reference.

• Issue: Tkinter extention modules have no documentation #59551

• Issue: Improve Tkinter Documentation #86726