Fix draggable annotations on subfigures.

[anntzer]

• Picking on subfigures was broken because ax = getattr(a, 'axes', None) would return the SubFigure.axes list when a is a subfigure, so the axes identity test would just be completely wrong. Fix that by special-casing subfigures. (Ideally I would entirely remove the axes equality test because that also breaks in the case of overlapping axes, but that's a much bigger change.)
• ref_artist.figure._canvas_callbacks would fail when the ref_artist is on a subfigure because subfigures don't have a _canvas_callbacks attribute; instead use ref_artist.figure.canvas.callbacks where accessing the canvas attribute correctly goes back to the toplevel figure first.

See test for an example (which would previously fail with an AttributeError).

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[QuLogic]

There's a existing issue #25860 for the first part, and a PR #26916, though I've not compared it with this one.

[anntzer]

From a quick look I think I still prefer my approach (#26916 compresses two recursion levels of the artist tree walking into a single one when the first level would incorrectly cause an entire subtree (associated with a SubFigure) to be skipped, whereas this PR explicitly disables subtree-skipping with SubFigures).