Add legal section for icons in AboutWindow

Fixes #31 (closed) and supersedes !21 (closed)

I'm not entirely sure whether the license is correct though as there is basically no information available...

Btw. the same is true for this website: https://teams.pages.gitlab.gnome.org/Design/icon-development-kit-www/

Missing legal information about icons seems to be a problem in general for GNOME icons.

Result