Refactor SimulationOptionsItem (no SessionItem, no SessionModel any more)

SimulationOptionsItem has been freed from infrastructure of SessionItem and SessionModel. It became a quite simple class, where no hidden magic is running behind the scenes.

There have been done a lot of preparing refactorings, but the main change took place in SimulationOptionsItem.h / .cpp (in commit eb5c80f1)

Also note that the wrapping DocumentModel (derived from SessionModel) was eliminated completely.