Change the way grades are handled fundamentally.

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.

svn path=/trunk/KDE/kdeedu/libkdeedu/; revision=770972