separating mkVariadic to implementation and function exposed to js

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
moving c++ extensibility support code to the cutes library

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[devel] cutes-devel package to develop cutes extension libraries

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>