High-level abstractions for `engine` API

Some patterns in the code-generated `engine` API are relatively frequent and could benefit from thin higher-level wrappers.

This issue tracks such ideas, please add one post for each idea 🙂

**Please don't start implementing those just yet.**

I'd like to keep this for a bit later, when we have a clearer idea of common use cases. We could then also decide if such features are in scope for gdext (and where they'd fit best), or if they could be done in an external project, for example.