Add a method to BView::Private that calls BView::_RemoveSelf().

Add a struct to kits/interface/Layout.cpp that uses BView::Private to
provide RAII mechanics for the removal of views (calling
BView::Private::RemoveSelf()).
Use the new struct to simplify BLayout::AddItem().

headers/private/interface/ViewPrivate.h

@@ -66,6 +66,11 @@ public:
 	bool	WillLayout();
 	bool	MinMaxValid();
 
+	bool RemoveSelf()
+	{
+		return fView->_RemoveSelf();
+	}
+
 	BView* fView;
 };