The text format definition needs to stay in `scene/` as it relies
on other scene components, and `core/` must stay self-contained.
This reverts commits a5e27503fd
and 1492fd8460.
-Scene edit state is saved outside the scene now, to avoid changes .tscn files when nothing really changed
-Created a VariantWriter helper to unify all variant to text writing
-Moved SceneFormatText writing to VariantWriter
-Moved ConfigFile to use VariantWriter and VariantParser, added compatibility mode for old .cfg files that use engine.cfg format
