Fix race condition between previous and next weather request

Once a weather request was finished, its worker gets destroyed automatically
but not immediately. This can cause it to still exist, when the next
request is being processed.
Therefore wait with sending the fetchingFinished() signal, until the
worker is fully destroyed.

ref T235

src/plugins/weatherdata/gfs/weatherdatagfs.cpp

@@ -98,7 +98,9 @@ namespace BlackWxPlugin
 
         void CWeatherDataGfs::fetchingWeatherDataFinished()
         {
-            emit fetchingFinished();
+            // If the worker is not destroyed yet, try again in 10 ms.
+            if (m_parseGribFileWorker) { QTimer::singleShot(10, this, &CWeatherDataGfs::fetchingWeatherDataFinished); }
+            else { emit fetchingFinished(); }
         }
 
         void CWeatherDataGfs::parseGfsFile(QNetworkReply *nwReplyPtr)