- Fix library loading and initialization.
- Add extra methods/parameters in the interface needed by extenstions.
- Add Variant destructors and functions for extracting values and
creating Variants from values.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
* Deprecates GDNative in favor of a simpler, lower level interface.
* New extension system allows registering core engine classes.
* Simple header interface in gdnative_interace.h