qt: Get rid of most uses of QString::asprintf()

Now that the Win32 UI is out of the way, we can rewrite the strings to use
QString::arg() instead and avoid even more QString ↔ C string conversions

src/qt/qt_settingsmachine.cpp

@@ -69,7 +69,7 @@ SettingsMachine::SettingsMachine(QWidget *parent)
     waitStatesModel->setData(idx, 0, Qt::UserRole);
     for (int i = 0; i < 8; ++i) {
         idx = waitStatesModel->index(i + 1, 0);
-        waitStatesModel->setData(idx, QString::asprintf(tr("%i Wait state(s)").toUtf8().constData(), i), Qt::DisplayRole);
+        waitStatesModel->setData(idx, tr("%1 Wait state(s)").arg(i), Qt::DisplayRole);
         waitStatesModel->setData(idx, i + 1, Qt::UserRole);
     }