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]>

docs/user/interface/TabView.dox

@@ -140,6 +140,8 @@
 /*!
 	\fn const char* BTab::Label() const
 	\brief Returns the tab's label (the target view's name).
+	\warning In BeOS, the tab label is determined from the view name. In Haiku, they can be
+		different if SetLabel() was used.
 
 	\since BeOS R3
 */
@@ -147,7 +149,9 @@
 
 /*!
 	\fn void BTab::SetLabel(const char* label)
-	\brief Sets the target view's name, and updates the BTab accordingly.
+	\brief Sets the tab label.
+	\warning In BeOS, setting the tab label also renamed the view attached to the tab. In Haiku,
+		this is not the case anymore, the label and view name are independant.
 
 	\since BeOS R3
 */
@@ -220,6 +224,9 @@
 /*!
 	\fn void BTab::SetView(BView* view)
 	\brief Sets the view to be displayed for this tab.
+
+	This also resets the tab label to match the view name (to preserve BeOS behavior).
+	If you need to use a different label, SetLabel must be called after SetView.
 */