a few highlights:
* BLayout now derives publicly from BLayoutItem
* Added BAbstractLayout class, which our layouts now derive from
* updated layout builders to avoid creating views when they don't need to
* updated layout classes
* updated AboutSystem to fix a little regression
* more details on #6407
* please tell me about any regressions, I've tried to find them all, but some
may have slipped by.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@38207 a95241bf-73f2-0310-859d-f6bbb57e9c96
