[Feature] Prefab system

[brenocq]

Feature description
Currently, an Atta project defined in a .atta file stores all its entities and components within that single file. This makes it difficult to reuse common configurations of entities and components (like a complete robot model, a specific environment setup, or a piece of furniture) across multiple projects without copy-pasting, which is error-prone and hard to maintain.

Game engines often solve this with a "prefab" system. This feature proposes implementing a similar system in Atta, allowing users to define reusable sets of entities and components in separate files (e.g., .apfb) and then include or instantiate them within a main .atta project file.

When Atta's file module loads a .atta file containing a reference to such an external file, it should load the contents of the included file as if they were defined directly within the main file, effectively instantiating the prefab/scene part.

This would enable:

• Reusability: Easily reuse complex objects or setups across different projects.
• Organization: Break down large scenes into smaller, manageable, modular files.
• Collaboration: Allow team members to work on different parts of a scene independently.
• Future Potential:
  • Create libraries of standard prefabs (robots, sensors, environments).
  • Build converters from standard formats like URDF or USD into this reusable Atta format (.apfb).

Tasks

• Design the syntax/mechanism within the .atta file format to specify the inclusion of an external prefab/scene file (e.g., specifying a file path).
• Decide on the standard file extension(s) for these reusable files (e.g., .apfb).
• Modify the .atta file loading logic (likely involving the file module and serialization systems) to detect and process these inclusion directives.
• Implement the instantiation logic: when an inclusion is processed, create the necessary entities and components as defined in the external file within the context of the main scene.
• Define how relative paths for included files should be resolved.
• Address potential issues with naming conflicts or unique ID generation when instantiating multiple copies of the same prefab.
• (Optional) Add UI functionality to select a set of entities in the Scene Tree and save them as a new prefab file (.apfb).
• (Optional) Integrate prefab browsing and insertion with the File System Browser ([Feature] File System Browser Window).
• Document the file format syntax for inclusions and the usage workflow.
• Test thoroughly with various scenarios, including nested inclusions and multiple instantiations of the same prefab.