Refresh add-in icons

[stakats]

They're OK but could use an updated look, and most (all?) have some fuzziness on a Retina display. This is not a high-priority item.

[adomasven]

FWIW, the add-in icons have been updated half a year ago. The fuzziness is not something that we have tools to address with at the moment. There's no customUI spec for hidpi images.