The custom multi widget allows to change the layout of the shown render windows (e.g. adding a new render window). Other parts of the workbench might need to know which render windows are currently available (e.g. the render window manager). How to the multi widget (editor) inform other workbench parts?
Currently the RenderWindowPartActivated-function of an implementation of the IRenderWindowPartListener is used inside the QmitkRenderWindowManagerView. However, this function is only called if the render window parts is initialized (e.g. the stdmultiwidget is newly opened).