Cleanup in load() functions
- reduced the number of i18n strings in the classes by providing central function in XmlStreamReader
- consistently handle unknown XML elements in all classes
- print out the line in the XML file for unknown elements
- In case the loading of a child in CartesianPlot::load() fails, no need to remove the child from the list of child aspects since it was not added to this list yet, deleting the object is enough.