Remove SessionItem-related methods and mechanism from RealDataItem (Major change)

All SessionItem-related mechanism and data-structures are removed from RealDataItem.

The following 'children tags' (SessionItemTags) are replaced by their simpler explicit equivalents:

* P_INSTRUMENT_ID => QString
* T_INTENSITY_DATA => std::unique_ptr<DataItem>
* T_NATIVE_DATA => std::unique_ptr<DataItem>
* P_NATIVE_DATA_UNITS => QString

To serialize/deserialize the data members, the methods RealDataItem::writeNonSessionItems and RealDataItem::readNonSessionItems are implemented.

NOTE: It is not clear if 'Native Data' is still needed or used; yet I avoided discarding this member at this stage of refactoring.

Related to issues #89 (closed), #90 (closed)