[FEATURE] Allow to destroy SceneModel but retain it's ViewObjects in the Viewer

[xeolabs]

Have option to make SceneModel transient.

Currently, when we create a SceneModel in the Scene, the Viewer will automatically create ViewObjects to represent the SceneModel's SceneObjects. When we then destroy the SceneModel, the Viewer will automatically destroy the ViewObjects.

Add an option to retain the ViewObjects when we destroy a SceneModel. This way, the SceneModel has the option to be used as a transient container of content to load into a Viewer. This would be useful when we want to be efficient with memory usage in the JavaScript process.