Add qapplication

[mneilly]

cxx-qt-lib contains qcoreapplication and qguiapplication but not qapplication. QCharts requires the use of QApplication so it would be desirable to have it added.

[ahayzen-kdab]

As QApplication comes from the widgets module of Qt, I think where we end up putting QApplication should be included in the cxx-qt-lib-extras discussions in #766 . Eg which modules and types from Qt do we determine to be fully supported and useful and which are community maintained or less stable etc

I think having at least some of the base types to help write a Qt widgets application with cargo + rust would be useful but we need to be careful not to have to try and maintain wrappers around the whole of the Qt widgets API.