improve canvas fit for Nautilus/save icons

- match the grid fit of the highres on the 48x48
  to avoid different scale at different viewport sizes

https://bugzilla.gnome.org/show_bug.cgi?id=724511
Addon icon

- pseudo mimetype because I couldn't think of anything better

https://bugzilla.gnome.org/show_bug.cgi?id=724669
repositories

- again, mimetype because I can't think of anything more appropriate

https://bugzilla.gnome.org/show_bug.cgi?id=724669