Big change deriving BLayout from BLayoutItem, and allowing viewless BLayouts.

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

headers/os/interface/Window.h

@@ -103,6 +103,7 @@ public:
 			void				Close() { Quit(); }
 
 			void				AddChild(BView* child, BView* before = NULL);
+			void				AddChild(BLayoutItem* child);
 			bool				RemoveChild(BView* child);
 			int32				CountChildren() const;
 			BView*				ChildAt(int32 index) const;