Core: MainWindow code cosmetics

Modernize connections, namespace Utils, include order.

Change-Id: Ic2e414dd4fb55821c6314bfc5256ce166fd70c2e
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/coreplugin/mainwindow.h

@@ -136,7 +136,6 @@ protected:
 private slots:
     void openFile();
     void aboutToShowRecentFiles();
-    void openRecentFile();
     void setFocusToEditor();
     void saveAll();
     void aboutQtCreator();