Dependency cycle between QtDatavisualization and QtDatavisualizationQml

When trying an ubsan build of qtdatavis3d, I get:

qtdatavis3d/src/datavisualization/CMakeFiles/DataVisualization.dir/engine/abstract3dcontroller.cpp.o:(.data.rel+0x5838): undefined reference to `typeinfo for AbstractDeclarative'

Digging into it, I found that AbstractDeclarative is defined only in DataVisualization_Qml_, a separate library that depends on DataVisualization.

It seemed this worked because Abstract3DController only used inline members of AbstractDeclarative, so a normal build doesn't need the class exported.

But an UBSan build requires the type_info, which isn't available, and cannot be made available because of the layering violation.

To fix, AbstractDeclarative needs to be moved to DataVisualization and exported (it can't, being a QQuickItem, so that, in turn, means that the class needs to be split into an interface that can live in DataVisualization and an interface/implementation that has to live in DataVisualizationQml. iow: dependency inversion.