Fixes example placement in builds.

Moves the Canvas3D examples from main examples"folder in
the Qt builds to Canvas3D subfolder.

Change-Id: Ib033db671a6ac790a1d7f659bdb13755fe7806ea
Task-number: QTBUG-45948
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>