Gui: Add QtStyle user parameter

This allows theme developers to set proper Qt style.

src/Gui/PreferencePages/DlgSettingsGeneral.cpp

@@ -251,6 +251,9 @@ void DlgSettingsGeneral::saveSettings()
     int blinkTime{hGrp->GetBool("EnableCursorBlinking", true) ? -1 : 0};
     qApp->setCursorFlashTime(blinkTime);
 
+    std::string qtStyle = hGrp->GetASCII("QtStyle");
+    qApp->setStyle(QString::fromStdString(qtStyle));
+
     saveDockWindowVisibility();
 
     hGrp = App::GetApplication().GetParameterGroupByPath("User parameter:BaseApp/Preferences/MainWindow");