This commit changes the way C++ wrapper classes work.
Previously, wrapper classes were merely wrapper *interfaces*.
They used the `this` pointer to store the actual foreign Godot
Object.
With the NativeScript 1.1 extension it is now possible to have
low-overhead language binding data attached to Objects.
The C++ bindings use that feature to implement *proper* wrappers
and enable regular C++ inheritance usage that way.
Some things might still be buggy and untested, but the C++
SimpleDemo works with those changes.
e9517a7b3b