Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
- alloc_language_binding: Use strong GC handle as well for references. Fixes#15138
- Set the native instance field of Godot.Object to IntPtr.Zero when it's freed.
- Create weak handles without tracking resurrection (that was causing trouble). This means we have to call notification predelete before queueing a native Object for deletion, and use the MonoObject* passed by the finalizer because the weak GC handle target will return NULL at this point.
Ignacio Etcheverry
Rémi Verschelde