Add manual method, enum, property exporting

[migueldeicaza]

Add support for exporting individual methods back to Godot:

In Godot/C++ that is done with code like this:

ClassDB::bind_method(D_METHOD("get_id"), &ExampleRef::get_id);

Properties would allow to surface existing properties to Godot, and might be easier to do once Swift gets the new deflectable APIs.

[migueldeicaza]

Done