ContainerControl designer inspector missing root appearance properties

The ContainerControl designer inspector for a custom container file only showed Size, so BackgroundColour and ClipContents were unavailable when editing the container root. Users could still see those properties on a ContainerControl placed in a window, which made custom container design inconsistent.

Expected behaviour: opening a ContainerControl designer tab exposes BackgroundColour and ClipContents for the container root and persists them.