Remove JSClass macros from Ns, GType, and Cairo types

Follow-up from !530 (merged) (Repo), !533 (merged) (Param), and !537 (merged) (Importer). Those didn't actually need the JSClass macros at all. In Ns, GType, and the Cairo types, on the other hand, the macros do save us from repeating a lot of code. However, this can be done in a more type-safe way with templates. See the commit message of "js: Add new system for creating wrapper objects for native types" for more detailed explanation.

Function also needs this, but that will be done in a follow-up merge request.