Merge pull request matplotlib#18226 from QuLogic/widget-cbs

Use CallbackRegistry in Widgets and some related cleanup

Author: timhoffm

doc/api/next_api_changes/behavior/18226-ES.rst

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+Widgets use ``CallbackRegistry`` to save callbacks
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+`.Widget`\s with event observers now use a `.CallbackRegistry` for storing
+callbacks. This is consistent with canvas event callbacks, and fixes some bugs
+in widget callback handling.
+
+Due to this change, callback methods are now stored as weak references, which
+means you must keep a reference to the associated object. Otherwise it may be
+garbage collected.

doc/api/next_api_changes/deprecations/18226-ES.rst

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+Widget class internals
+~~~~~~~~~~~~~~~~~~~~~~
+
+Several `.widgets.Widget` class internals have been privatized and deprecated:
+
+* ``AxesWidget.cids``
+* ``Button.cnt`` and ``Button.observers``
+* ``CheckButtons.cnt`` and ``CheckButtons.observers``
+* ``RadioButtons.cnt`` and ``RadioButtons.observers``
+* ``Slider.cnt`` and ``Slider.observers``
+* ``TextBox.cnt``, ``TextBox.change_observers`` and
+  ``TextBox.submit_observers``