Adding dropdown menu to image segmentation tool example

[rkube]

I think your dropdown widget example is useful and I added it to the image-segmentation notebook.

[ianhi]

Thanks @rkube !

I pushed a fix for the function name being wrong. There also an error where the explanation cell doesn't seem to be a markdown. I think the other test failures are unrelated and/or external this library. (psf/black#2079 and matplotlib/matplotlib#19848)

[ianhi]

Thanks again! This will up on https://mpl-interactions.readthedocs.io/en/latest/examples/image-segmentation.html as soon as the build finishes and on the stable branch the next time I make a release.