this fixes#3200.
This causes permissions to be registered before plugin load, which changes some behaviour, but after discussions on #internals-dev we couldn't see any reason to keep the behaviour the way it was, and several reasons to change it to be something like this.
TextContainer provided zero real value as a base of TranslationContainer, given that it required its own logic to be handled wherever accepted. As such, it's no better than a simple string.
Removing it also allows fixing an ambiguity when embedding translations inside other translations, allowing it to be made immutable.
