Review: Merge of margin collapsing from WebCore

[Allan Sandfeld Jensen]

I've merged the new layout algorithm from WebCore-315.

Not only does it fix the gap in Acid2, but also several repported bugs (for 
instance the real-world case #103130).

At the moment however it produces two regressions in the regression suite:
1. Scrollbars do not appear for relative positioned overflowing content.
2. Definition list are not rendered quite compact, and in one case even 
crashes (unsorted/750806.html).

http://www.carewolf.com/margins.diff

`Allan