Rm lots of obsolete code; free RealSpaceBuilder from SessionItem

Main point is to free RealSpaceBuilder from SessionItem knowledge. For this, the variant SampleItem has been introduced. It can keep any item class ptr without the need to have an artificial common base class for all items (namely SessionItem). Right now only a few item ptrs are possible (only the necessary ones). This will be enhanced as needed. It even may be a temporary solution until the SampleModel refactoring is finished.

Besides this, a warning (only shown in Windows builds) and obsolete code has been removed