Gui: Use new-style syntax of connect()

When fixing clazy issues -Wclazy-fully-qualified-moc-types then old-style syntax of connect() may fail. Thus, replace it with the new-style syntax

src/Gui/MDIView.h

@@ -36,6 +36,7 @@ QT_END_NAMESPACE
 namespace Gui
 {
 class Document;
+class MainWindow;
 class ViewProvider;
 class ViewProviderDocumentObject;
 
@@ -182,6 +183,8 @@ private:
     ActiveObjectList ActiveObjects;
     typedef boost::signals2::connection Connection;
     Connection connectDelObject; //remove active object upon delete.
+
+    friend class MainWindow;
 };
 
 } // namespace Gui

src/Gui/MainWindow.cpp

@@ -876,10 +876,8 @@ void MainWindow::addWindow(MDIView* view)
         d->mdiArea->addSubWindow(child);
     }
 
-    connect(view, SIGNAL(message(const QString&, int)),
-            this, SLOT(showMessage(const QString&, int)));
-    connect(this, SIGNAL(windowStateChanged(MDIView*)),
-            view, SLOT(windowStateChanged(MDIView*)));
+    connect(view, &MDIView::message, this, &MainWindow::showMessage);
+    connect(this, &MainWindow::windowStateChanged, view, &MDIView::windowStateChanged);
 
     // listen to the incoming events of the view
     view->installEventFilter(this);
@@ -900,10 +898,9 @@ void MainWindow::addWindow(MDIView* view)
 void MainWindow::removeWindow(Gui::MDIView* view, bool close)
 {
     // free all connections
-    disconnect(view, SIGNAL(message(const QString&, int)),
-            this, SLOT(showMessage(const QString&, int )));
-    disconnect(this, SIGNAL(windowStateChanged(MDIView*)),
-            view, SLOT(windowStateChanged(MDIView*)));
+    disconnect(view, &MDIView::message, this, &MainWindow::showMessage);
+    disconnect(this, &MainWindow::windowStateChanged, view, &MDIView::windowStateChanged);
+
     view->removeEventFilter(this);
 
     // check if the focus widget is a child of the view