Google Code-in

Follow up on https://bugs.kde.org/show_bug.cgi?id=339839.

This is a bit more difficult; we need to touch ProblemModel/FilteredProblemModel/ProblemStore as far as I can see.

Implementation hint:

• Introduce a flag for severity: http://doc.qt.io/qt-5/qflags.html ProblemModel::setSeverity(int severity) will turn into ProblemModel::setSeverity(Severities severities) Code around ProblemModel::FeatureCode is very similar to this, you can use this as reference
• Start implementing the non-UI parts of feature by adding/modifying tests in TestFilteredProblemStore!
• Then fix the UI and connect the buttons to the proper methods in ProblemModel