v0.2.4: UPBGE: Fix duplication of navigation mesh objects.

The duplication of navigation mesh was processing a regeneration of the mesh
and a registration to the scene in the function BuildNavMesh. But the caller
(ProcessReplica) is on an object without a scene graph node and so an access
to its scene, this mean that the try to register the nav mesh in the scene
obstacle simulation failed in BuildNavMesh because the scene is unreachable.

To fix this issue the management of obstacle simulation for nav mesh and
obstacles is fully moved into KX_Scene::AddNodeReplicaObject after the new
object ensure a scene graph node.

Fix issue #926.