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/View.h

@@ -562,6 +562,8 @@ public:
 			void				Layout(bool force);
 			void				Relayout();
 
+	class Private;
+
 protected:
 	virtual	void				DoLayout();
 
@@ -580,6 +582,8 @@ protected:
 
 private:
 			void				_Layout(bool force, BLayoutContext* context);
+			void				_LayoutLeft(BLayout* deleted);
+			void				_InvalidateParentLayout();
 
 private:
 	// FBC padding and forbidden methods
@@ -595,6 +599,7 @@ private:
 private:
 	struct LayoutData;
 
+	friend class Private;
 	friend class BBitmap;
 	friend class BLayout;
 	friend class BPrintJob;