Support static methods for boxed and structure types

Expose all methods that don't have GI_FUNCTION_IS_METHOD flag set
as static methods on the JS class constructor. Currently, for boxed,
struct and variant types, only C constructors (class_new_foo) are
associated, other static methods are left as functions in the global
namespace.

https://bugzilla.gnome.org/show_bug.cgi?id=622344