ui: Disable unused QToolBar context menu

- Right clicking on the QToolBar invoked a context menu
hiding the said toolbar when clicked on.

Change-Id: Ifbc86ce34d508bb7348ba5ca68ee3ef324e46ad7
Tuleap: #148

mainwindow.cpp

@@ -106,6 +106,8 @@ MainWindow::MainWindow(QWidget *parent) :
         win_sparkle_cleanup();
     });
 #endif
+
+    setContextMenuPolicy(Qt::NoContextMenu);
 }
 
 MainWindow::~MainWindow()