Debugger: Move perspective saving to a time where mode is active

It should not make and I do not observe a difference, but it seems
the better thing to do.

Change-Id: I1f1f9fba72c2b20f24ebde04cf4ec3711d77d476
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/debugger/debuggermainwindow.h

@@ -126,7 +126,8 @@ public:
     static void doShutdown();
 
     static void showStatusMessage(const QString &message, int timeoutMS);
-    static void onModeChanged(Core::Id mode);
+    static void enterDebugMode();
+    static void leaveDebugMode();
 
     static QWidget *centralWidgetStack();
     void addSubPerspectiveSwitcher(QWidget *widget);