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/GroupLayoutBuilder.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006, Haiku, Inc. All rights reserved.
+ * Copyright 2006-2010, Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  */
 #ifndef	_GROUP_LAYOUT_BUILDER_H
@@ -39,7 +39,6 @@ public:
 									float bottom);
 
 								operator BGroupLayout*();
-								operator BView*();
 
 private:
 			bool				_PushLayout(BGroupLayout* layout);