This will be used in subsequent commits. https://bugzilla.gnome.org/show_bug.cgi?id=760536
This is sometimes needed, and calling into actual icon theme code just for it is confusing - the resulting icon does not depend on the icon theme at all. https://bugzilla.gnome.org/show_bug.cgi?id=760536
This is the first step in replacing StyleContext usage with CssNode usage.