Add oxygen-gtk3.

GTK 3 version of the Oxygen theme.

Because the GTK2 version is called `oxygen_gtk` I've added a `oxygen_gtk3` as well as a `oxygen-gtk3`.  I think the latter is preferred now but the former should be there as well to make it more consistent.  But I don't know what the official position on `_` vs `-` is and right now it's a bit of a mess to be honest.

/cc @cillianderoiste 
Sorry, I missed this when I first had a look. Please add yourself to lib/maintainers.nix and here. Thanks!