Merge pull request matplotlib#22316 from anntzer/toolbarhooks

Arbitrary figure customization hooks.

Author: ksunden

doc/users/next_whats_new/figure_hooks.rst

@@ -0,0 +1,8 @@
+Figure hooks
+~~~~~~~~~~~~
+The new :rc:`figure.hooks` provides a mechanism to register
+arbitrary customizations on pyplot figures; it is a list of
+"dotted.module.name:dotted.callable.name" strings specifying functions
+that are called on each figure created by `.pyplot.figure`; these
+functions can e.g. attach callbacks or modify the toolbar.  See
+:doc:`/gallery/user_interfaces/mplcvd` for an example of toolbar customization.