Debugger: Have main window handle mode change separately

On shutdown the DebuggerMainWindow might not be alive anymore. By
connecting the signal handler to the window, rather than the plugin we
avoid dereferencing null pointers in that case.

Change-Id: I6421eebc6b1b76f176b31eafeb039d91edd3f0b3
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggermainwindow.h

@@ -129,6 +129,8 @@ public:
     QByteArray currentPerspective() const { return m_currentPerspectiveId; }
     QStackedWidget *centralWidgetStack() const { return m_centralWidgetStack; }
 
+    void onModeChanged(Core::Id mode);
+
 private:
     QDockWidget *registerDockWidget(const QByteArray &dockId, QWidget *widget);
     void loadPerspectiveHelper(const QByteArray &perspectiveId, bool fromStoredSettings = true);