so the components like scale and foobar should be registered into the
panel-component (compositum) and displayed in the panel's content-group.
how to achieve this?
I've seen one possible way of doing this. The Nebula PShelf control
contains PShelfItems, which have a method getBody() that returns a
composite. You could add this method to your composite, but you would
have to document very prominently that all children should be created
using panelComposite.getBody() as the parent. This may be difficult to
enforce in practice.
Another approach you might consider is to let users create their
controls directly on the PanelComposite, but force them to call
setContent with that widget. Then you just override the layout() method
of PanelComposite and only perform layout on that one widget.