Update godot_export_manager.py

- Added the option to apply the scale, rot and loc of objects before exporting
- Added support for group instances in blender. This has a limitation currently. All group instances will get their own data. They don't share mesh or armature data currently. This is because there are import problems in godot currently.
- Cleaned up the code a bit. Before I saved objects selection and that stuff and later reconstructed that manually. Now I just undo everything after export and then push a new undo message when everything is done. Makes the code cleaner and works perfeclty.