replace QTime with QElapsedTimer when possible (qt6)

Since Qt5, some methods of QTime [are deprecated](https://doc.qt.io/qt-5/qtime-obsolete.html).

Qt6 remove those methods, thus in two instances we need to use
a [QElapsedTimer](https://doc.qt.io/qt-6/qelapsedtimer.html) instead of a QTime.

QElapsedTimer is available since Qt5.4, and is already used in the source
code.

mainwindow.cpp

@@ -177,7 +177,7 @@ MainWindow::MainWindow(QWidget *parent) :
     updateTimer.setInterval(250);
     updateTimer.start();
 
-    elapsedTime = new QTime;
+    elapsedTime = new QElapsedTimer;
     elapsedTime->start();
 
     isConnected = false;