[MusicXML] element offset handled incorrectly

The MusicXML offset element is handled incorrectly: it is read into int _mxmlOff in class Element, but not actually used for layout. It should be translated in a corresponding user offset in x direction instead and _mxmlOff should be removed. On export _mxmlOff is incorrectly written as offset to MusicXML, while ignoring the actual position offset.