Refactor titleformats and titlespacings

[wojtask]

At present we use different specification for titleformat and titlespacing for section and subsection depending on where they appear - in the main chapter area or in the problems area. The commands \chapter and \problem modify these specifications, while in order to reuse \section and \subsection levels.

The titlesec package supports defining new levels and changing their classes (section 3.9). Find out if creation of new levels, for problems and subproblems, could simplify the configuration, without the confusion of reusing the same levels.