itom2dqwtplot: value picker shows wrong value

If the value picker is enabled in itom2dqwtplot, the value is constantly 0 for fixed-point numbers and nan for floating-point numbers. The bug seems to be in the method

double DataObjRasterData::value(double x, double y) const

especially in the first switch case, since a comparison of m_dataObj.getType() with the enumeration value inPlane is probably errorneous.