Prevent precise timers from being used when unnecessary

The implementation of QTimer::singleShot() uses Qt::PreciseTimer if interval is less than 2 seconds. This isn't mentioned in the docs.
Qt::PreciseTimer increases the system's timer resolution which negatively affects power consumption.

PR #18555.
Closes #18350.

src/webui/api/appcontroller.cpp

@@ -94,7 +94,7 @@ void AppController::shutdownAction()
     // Special handling for shutdown, we
     // need to reply to the Web UI before
     // actually shutting down.
-    QTimer::singleShot(100ms, qApp, []()
+    QTimer::singleShot(100ms, Qt::CoarseTimer, qApp, []
     {
         QCoreApplication::exit();
     });