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/private/interface/ViewPrivate.h

@@ -16,6 +16,7 @@
 #include <Rect.h>
 #include <Region.h>
 #include <ServerProtocolStructs.h>
+#include <View.h>
 
 const static uint32 kDeleteReplicant = 'JAHA';
 
@@ -48,6 +49,27 @@ enum {
 };
 
 
+class BView::Private {
+public:
+	Private(BView* view)
+		:
+		fView(view)
+	{
+	}
+
+	int16 ShowLevel()
+	{
+		return fView->fShowLevel;
+	}
+
+	// defined in View.cpp
+	bool	WillLayout();
+	bool	MinMaxValid();
+
+	BView* fView;
+};
+
+
 namespace BPrivate {
 
 class PortLink;