Replace string-referencing by real methods

In the current session model many referencing is done by strings:

E.g.:

realDataItem->getItem(RealDataItem::P_NATIVE_DATA_UNITS)->setValue(units);

This is quite unsafe regarding refactoring and very inconvenient to follow references with IDEs. Also the compiler is not able to check such references.

It should be replaced by real methods, which in a first step just wrap the former call:

void RealDataItem::setNativeDataUnits(const QString& units)
{
    getItem(P_NATIVE_DATA_UNITS)->setValue(units);
}

realDataItem->setNativeDataUnits(units);