Custom3DItem should use QGeometryRenderer

Custom3DItem builds from an OBJ file that contains a specific type of Mesh.

It would be much better if Custom3DItem could build from QGeometryRenderer. 

It would then be possible to overlay, on any QAbstract3DGraph, any type of the following

Qt3DExtras::QConeMesh, Qt3DExtras::QCuboidMesh, Qt3DExtras::QCylinderMesh, Qt3DExtras::QExtrudedTextMesh, Qt3DExtras::QPlaneMesh, Qt3DExtras::QSphereMesh, Qt3DExtras::QTorusMesh, and Qt3DRender::QMesh

An example use case is to draw a semi transparent ellipse over a Q3DScatter set of points to illustrate what PCA does.