[TASK] Clean up language files

[rintisch]

• Every language files has it's clear domain.
• No unused entries in the files.
• Some labels which are used in the backend are used from the locallang.xlf but have their own entries which are namespaced with e.g. tx_cart.backend.*.

This PR build on #465. I will rebase it after #465 was merged.

Fixes #471