Change the way grades are handled fundamentally.
Too bad, I didn't have the idea long ago.

Much simpler:
* One grade for each word, no longer the fromTranslation.
So whenever the user gets abc right, abc gets it's grade increased.
* Translation inherits from the new Text class.
* The new class KEduVocText will be used instead of QString in many places to for ex allow grades for individual conjugation forms etc.
* Add initial unittests for translation class.