Fix faulty number of new words counting

Summary:
The configurable number of new words per practice session was not
counted correctly for gender, conjugation and comparison practice.
The above mentioned practice modes uses separate (pre-)grades for the
special word type subsets. These were not recognized by the old
counting algorithm. An improved practice mode dependent (pre-)grade
detection was introduced.

Reviewers: #kde_edu, aacid

Reviewed By: aacid

Subscribers: aacid, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D17180