It seemed I made a wish of some people from the usability folks come true. Presenting: my first approach of a CollapsibleWidget!
The same dialog with two filled and one empty collapsable item on Linux, OS X and Windows:
Interesting to see that there are so many differences in the layouting among the different platforms -- odd. But I will sort them out eventually. On the positive side of things, this allows us to finally get rid of the tab-desert in most kde applications and separate common settings from rarely used ones.
This is the part of the usability guys: It's your turn again :).
PS: Whoever finds typos may keep them ;).