Add "Qt 3D: Materials" C++ example back to 5.12

In 5.10, there was a very helpful example showing how to use many aspects of Qt3D in C++. For some reason this was removed in 5.11 and 5.12. I tested it on 5.12 and it still works. It would be very helpful for future reference if this was added back to the code base.

https://doc.qt.io/archives/qt-5.10/qt3d-materials-cpp-example.html