Make it possible to create custom Item3D's in C++ QML plugins

Third party QML plugins may need to create specialized QML/3D items that inherit from Item3D. Move this class into a new libQt3DDeclarative.so library that can be accessed by such plugins. As an example of how to use it, implement the Sphere item in the Qt3D.Shapes namespace, dynamically generating the sphere based on level-of-detail.