BTab: decouple display label from view name.

In BeOS, the tab label was synchronized with the view name. This means
BTab::SetLabel would change the view name. This behavior is unexpected,
and also annoying, for example it prevents using localized tab labels
but fixed view names for scripting.

* Document this divergence from BeOS in the Haiku book
* Detect if we're running in a BeOS app to preserve the old behavior
  then

Change-Id: I5b758a035fe8752cee2ea7789a30ea47a01467b9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/122
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: Jérôme Duval <[email protected]>

headers/os/interface/TabView.h

@@ -82,8 +82,13 @@ private:
 			bool				fFocus;
 			BView*				fView;
 			BTabView*			fTabView;
+			BString				fLabel;
 
-			uint32				_reserved[11];
+#ifdef B_HAIKU_64_BIT
+			uint32				_reserved[9];
+#else
+			uint32				_reserved[10];
+#endif
 };