This also changes Reference::unreference() to always invoke
refcount_decremented. Previously it was not invoked until the count
reached zero due to short-circuit evalution of boolean expressions.
This adds GDNative as a separate class type.
It can be used to interface with native libraries by
using "native calls", which can be registered by modules
(and in future other GDNative libraries?).
It also reworks the currently called "GDNativeScript" into a
"NativeScript" that just makes use of the new GDNative instead
of it being the component that implements that functionality.
