Allow related changes list plugins to trigger visibility computation
If plugins loaded their content late, they had no chance to trigger
visibility computation of the related changes list. This made it
impossible for plugins that received their data late and hence
turned visible only late to make the whole related changes list
visible, if the related changes list would be hidden otherwise.
The control now listens to the new-section-loaded event which
also gets sent out by the control itself upon updates and is
respected by parents as well.