Add accessories-screenshot-tool icon/symlink

This supports the proposed XDG icon theme spec for it at https://gitlab.freedesktop.org/xdg/default-icon-theme/-/merge_requests/4.

Currently it's a symlink to Spectacle's icon, but we could easily reverse that if needed, and eventually change Spectacle's own icon to accessories-screenshot-tool in the future if and when this spec change gets implemented by more icon themes.