Inequality tasks on the second level + QP solver

[asherikov]

It is possible to solve a hierarchy of two levels with inequality tasks on the second level as a QP. This can be done by introducing slack variables explicitly and pushing the inequalities to the first level. Some architectural changes are necessary for this to work: weighting of the tasks must be handled differently, handling of solution vector must be changed, etc.

We hardly need this feature, consider this issue as a note.