Return TranslationWrapper in t() and $this->t() directly

Problem/Motivation

Now that TranslationManager::translate() no longer translates, but rather returns
a TranslationWrapper we could simplify the call chain of t() and $this->t()
quite a lot.

Proposed resolution

convert t() and $this->t() to return a TranslationWrapper

Remaining tasks

User interface changes

API changes

Data model changes