TCastleWindow.SceneManager: removal of a deprecated property soon

[DP News-Robot]

As we extend the engine, we sometimes have to deprecate some old features, that were not very useful, and sometimes prevented the engine API from being flexible and easy to understand. One of these things was TCastleSceneManager class (renamed to TCastleViewport since 2 years), with its single “central” instance in TCastleWindow.SceneManager or TCastleControl.SceneManager. In a …