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/LayoutItem.h

@@ -16,7 +16,7 @@ class BLayout;
 class BView;
 
 
-class BLayoutItem: public BArchivable {
+class BLayoutItem : public BArchivable {
 public:
 								BLayoutItem();
 								BLayoutItem(BMessage* from);
@@ -46,7 +46,8 @@ public:
 
 	virtual	BView*				View();
 
-	virtual	void				InvalidateLayout();
+	virtual	void				InvalidateLayout(bool children = false);
+	virtual	void				Relayout(bool immediate = false);
 
 			void*				LayoutData() const;
 			void				SetLayoutData(void* data);
@@ -57,11 +58,19 @@ public:
 	virtual status_t			AllArchived(BMessage* into) const;
 	virtual	status_t			AllUnarchived(const BMessage* from);
 
-private:
-			friend class BLayout;
+protected:
 
 			void				SetLayout(BLayout* layout);
 
+	// hook methods
+	virtual	void				AttachedToLayout();
+	virtual	void				DetachedFromLayout(BLayout* layout);
+
+	virtual void				AncestorVisibilityChanged(bool shown);
+
+private:
+			friend class BLayout;
+
 			BLayout*			fLayout;
 			void*				fLayoutData;
 };