Remove utf8cpp 3rd party lib from Qt 3D Studio

Utf8cpp seems to be only used by SimpleXmlWriter struct that has two copies in Studio code:

src/Authoring/UICDM/Systems/UICDMXML.cpp

src/Runtime/Source/Qt3DSFoundation/Source/foundation/XML.cpp

Should be possible to replace those use cases with Qt's xml functionality (http://doc.qt.io/qt-5/qxmlstreamreader.html and http://doc.qt.io/qt-5/qxmlstreamwriter.html).