Fully translate aria-labels for predefined backgrounds

[Antreesy]

☑️ Resolves

• Fix Aria label key words of virtual backgrounds are not translatable #9610
• Instead of translating Select virtual {{unknown}} background' and eight single-word strings like abstract or beach, translators now need to translate eight meaningful strings (which could take up to 3-5 minutes with Transifex autocompletion)
• Method of extracting filename without extension should work as before, according to JS prototypes behavior:
  • String.split(substr) always returns an array, even if substr isn't found;
  • Array.pop() and Array.shift() return last / first element of array, and the only existing element from 1-length array.

🖼️ Screenshots

🏡 After

🚧 Tasks

• Code review
• Visual check

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not required
• 📘 API documentation in docs/ has been updated or is not required
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required
• 🔖 Capability is added or not needed

[Antreesy]

/backport to stable27