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.