icons: add various other sizes of PNG icons.

For whatever reason, after updating to xfce-4.14, my SVG icon isn't
being used in contexts where it should. For example, I'm seeing a
"missing icon" image on the plugin's "about" dialog, where the
scalable icon would be a perfect choice.

The SVG icon is still there, and should be used as a last resort, but
maybe other people would experience this problem as well. To work
around it, I've manually created a bunch of PNG icons in various
sizes. This fixes my problem, and can't hurt anything (ha ha).