This allows to include script_instance.h directly in the
generated gdvirtual.gen.inc, and remove excessive includes
from the codebase.
This should also allow Resource to use GDVIRTUAL macros,
which wasn't possible previously due to a circular dependency.
- Simplify and update its logic.
- Simplify EditorScript.
- Improve EditorNode and other relevant includes.
- Fix scene-based path in the movie writer when
reloading a scene.