Fix loading user-defined icons for Qt plot window

[timhoffm]

PR Summary

Closes #22131.

I'll refrain from writing a test for this because that would be quite cumbersome. One can manually test the correctness be deleting one of the _large icons. Then the plot window falls back to the low-res version of that icon.
It's also unlike to break again, because the comment should make future authors aware that we have to have the fall back for user icons.

[QuLogic]

This affects Tk as well, I think.

[oscargus]

Yes, here:

matplotlib/lib/matplotlib/backends/_backend_tk.py

Lines 647 to 649 in 80d4e23

	with Image.open(button._image_file.replace('.png', '_large.png')
	if size > 24 else button._image_file) as im:
	image = ImageTk.PhotoImage(im.resize((size, size)), master=self)

[daniilS]

I've copied the changes to the Tk backend in #22147

[tacaswell]

I am also in favor of @QuLogic 's suggestion, but OK to merge either way.

[timhoffm]

Thanks, I found a cleaner way of doing this due to @QuLogic's suggestion and @daniilS's PR for Tk (#22147).