TabView: Add TabViewPrivate class, actually fix non-layout SetView.

Thanks to Axel and Stephan for reviewing!

headers/private/interface/TabViewPrivate.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2015, Haiku Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ *
+ * Authors:
+ *		Augustin Cavalier <waddlesplash>
+ */
+#ifndef TABVIEW_PRIVATE_H
+#define TABVIEW_PRIVATE_H
+
+
+#include <TabView.h>
+
+
+class BTab::Private {
+public:
+								Private(BTab* tab)
+									:
+									fTab(tab)
+								{
+								}
+
+			void				SetTabView(BTabView* tabView)
+									{ fTab->fTabView = tabView; }
+
+private:
+			BTab* fTab;
+};
+
+
+#endif	/* TABVIEW_PRIVATE_H */