Debugger: Rename MainWindow::{set,add}SubPerspectiveSwitcher

We'll need another one for GammaRay, and it's practically already
working as 'add', not 'set'. So fix the mis-nomer.

Change-Id: I1713ced6c39082dae0121b8d670c4676807c1602
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/debugger/debuggermainwindow.h

@@ -128,7 +128,7 @@ public:
     static void onModeChanged(Core::Id mode);
 
     static QWidget *centralWidgetStack();
-    void setSubPerspectiveSwitcher(QWidget *widget);
+    void addSubPerspectiveSwitcher(QWidget *widget);
 
 private:
     DebuggerMainWindow();