[FEATURE] Support River keyboard layout backend

[alexaandru]

Submission checklist

• I have searched existing issues and confirmed this has not been requested before.
• I have checked existing pull requests for similar changes.

Feature type

Integration support

Feature summary

Support River keyboard layout backned so that the keyboard layout widget reflects the layout changes as well as be able to change them itself.

Motivation / use case

River has quite a few WMs that could benefit from this.

Proposed solution

I already have it working against v5.1.0: river-keyboard-layout-backend.patch

If you are open to this feature, I can submit a PR.

NOTE: I used a coding agent to help build the patch.

Alternatives considered

No response

References / related projects

No response

Additional context

No response