testing/pinta: new aport

Marking as RFC because I'm not sure if we want to add a GTK2.0 app. I could alternatively try packaging the WIP GTK3 branch, but that's probably not a good idea.

(Note: mono-addins cannot be built with both the gtk2 and gtk3 GUI packages enabled, or it will error out during build - it's either one or the other. Currently it's only used by pinta, so this shouldn't be too big of a problem.)