Added planet example

Added example that shows how three.js can be used with QtCanvas3D.

Documentation will be added in separate commit.

Change-Id: I9557fefebd4c1a749af95bef3952560b03b3320f
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>
Reviewed-by: Sami Makkonen <sami.makkonen@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>