Adapt to QLocale::percent() returning a QString in Qt 6

src/lib/slider.cpp

@@ -64,7 +64,11 @@ void Slider::setValueLabel(QLabel* label, const QString& format, bool hideIfDisa
                     mValueFormat.insert(++i, QLatin1Char('L'));
                     continue;
                 }
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
                 mValueFormat[i] = QLocale().percent();
+#else
+                mValueFormat.replace(i, 1, QLocale().percent());
+#endif
             }
         }
     }