fix presets menu hierarchy with equally named submenus

[zisoft]

Summary

gtk3 flattens GtkPopoverMenu into a single GtkStack and names each page after its item label, so equally named submenus under different presets collide and navigation follows the first one.
Solution is to give submenus unique names.

Referenced issue

fixes #22398

Checklist

• I have read CONTRIBUTING.md and the coding style.
• I have not merged master into the topic branch.
• The pull request is one logical change, and every commit compiles on its own.
• I ran the relevant tests: unit tests, src/tests/integration/ where the pixelpipe is touched, or darktable-cli as a headless smoke test.
• New user-visible strings use _(), new preferences are registered in data/darktableconfig.xml.in.
• A RELEASE_NOTES.md entry was added (only needed if fixing an issue in a release). Do not reference GitHub issues.

[lefth]

Thank you, this works!