Added examples on how to use FBOs with scene graph.

I also cleaned up the naming of the other scene graph examples a bit,
so that they sort together in the samples list and have a more obvious
naming scheme.

Task-number: QTBUG-29548
Change-Id: I455eacb02c06058a6d49e12e4f1813ec80b655f6
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>