QIcon::fromTheme do not find icons installed after the application start when there is a gtk theme chache

gtk cache may be  invalid after the application start,  although I can resolve it to clear gtk cache by

```
QIcon::setThemeSearchPaths(QIcon::themeSearchPaths());
```

According to measurements made for https://bugreports.qt.io/browse/QTBUG-65720

I see libqtxdg is resoved it by filewatcher to update the cache.