Add widget→View reverse lookup and app widget hierarchy

[dccote]

Summary

Two small introspection helpers for mytk:

• Base.view_for(widget) — given any tkinter widget, return the mytk View that owns it (or None). Each placement method (grid_into/pack_into/place_into) now stamps the View onto its widget as a plain .mytk_view instance attribute right after the widget is created.
• App.widget_hierarchy(widget=None) — return the tkinter widget tree as a nested dict (class/name/path/children) using only tkinter's built-in winfo_* methods. Defaults to the app root, so the common call is just app.widget_hierarchy(); pass a widget to root at a subtree.

widget_hierarchy lives on App rather than _BaseWidget because inspecting the whole app tree is an app-level concern, whereas view_for is genuinely widget-level.

Usage

from mytk import Base

tree = app.widget_hierarchy()                 # nested dict of the whole UI
some_widget = app.window.widget.winfo_children()[0]
Base.view_for(some_widget)                    # the mytk View that owns it

Tests

New mytk/tests/testWidgetHierarchy.py (7 tests) covers view_for after all three placement methods, the unknown-widget None case, and widget_hierarchy shape/default-root/custom-root behavior. Full suite: 511 passed, 8 skipped.

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