Made ast_optional non-destructive

[vojtref]

Description

Made ast_optional retrieve attributes from the attribute map non-destructively.

Additional Notes

Previous behaviour had the :timeout attribute (currently the only reused attribute) immediately discarded even when checking non-existent command handlers, with all subsequent uses defaulting to 200 ms (see linked issues below). Keeping optional attributes in the property map avoids this problem, and potential future issues with attribute reuse.
ast_required may, at present, be kept destructive, as no required attributes are reused. Making all retrieval non-destructive is, however, worth considering.

Fixes #715.
Fixes #756.
Fixes #965.

Checklist

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing