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/GridLayoutBuilder.h

@@ -16,9 +16,9 @@ public:
 								BGridLayoutBuilder(BGridView* view);
 
 			BGridLayout*		GridLayout() const;
-			BView*				View() const;
 			BGridLayoutBuilder& GetGridLayout(BGridLayout** _layout);
-			BGridLayoutBuilder& GetView(BView** _view);
+			BView*				View() const;
+			BGridLayoutBuilder&	GetView(BView** _view);
 
 			BGridLayoutBuilder& Add(BView* view, int32 column, int32 row,
 									int32 columnCount = 1, int32 rowCount = 1);
@@ -32,7 +32,6 @@ public:
 									float bottom);
 
 								operator BGridLayout*();
-								operator BView*();
 
 private:
 			BGridLayout*		fLayout;