OSMF SVN

Removing MediaContainerGroup. Exposing the inner layout renderer publicly in order to facilitate container nesting. Updating NestedMediaContainers sample accordingly.

Overriding display list management functions on MediaContainer to throw an exception on direct invocation. As a result, layout renderers are no longer capable of adding display objects onto a container, but this is resolved by LayoutRendererBase now emitting events requesting display object (un)staging or index adjustment. LayoutTargetSprite and MediaElementLayoutContainer have been updated accordingly.