Bugfix: StatisticsView doesn't display correct values

Summary:
The color bars in LessonStatisticsView show only the correct values
when the widget was just created. Changing practice language, mode or
direction doesn't change the displayed values.

Added missing method calls in StatisticsMainWindow slot methods.
Implemented improved word counting for gender, conjugation and
comparison practice, to provide correct statistics values for this
practice modes.

Removed wrongly displayed horizontal scroll bar in LessonStatisticsView.
Modified column width calculation, not to use a hard coded magic number,
which does not fit to plasma anymore. Use instead the viewport width of
the LessonStatisticsView widget.

BUG: 387602

Reviewers: KDE Edu, apol

Reviewed By: apol

Subscribers: apol

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D9187