Fix ProblemModel::addProblem() method
The patch fixes wrong call of QAbstractItemModel::beginInsertRows() method. Previous version's arguments match the 2-rows adding case but only one row is really added. As a result after adding some problems the Problems View tree widget also displays "dead items" - empty and not-selectable rows:
| -SomeProblem1 |
| -SomeProblem2 |
| - |
| - |
(cherry picked from commit 8ed3f123a844f02f44d08b021db7c4484098b425)