Use CApplication::isNetworkAccessible() wherever possible

Up to now we had a mixture of different methods to detect whether
network is connected, accessible or both. Even if they implied a
different meaning, they were all doing the same and internally
referred to almost the same code. Almost, since our implementation
of CNetworkUtils::hasConnectedInterface refers to code from Qt 4.4.
However, all of them returned a boolean status whether a network
interface is generally up or not. Instead of maintaining our own
version, use Qt functions.

refs #869

src/plugins/weatherdata/gfs/weatherdatagfs.cpp

@@ -65,7 +65,7 @@ namespace BlackWxPlugin
             m_maxRange = range;
             if (m_gribData.isEmpty())
             {
-                if (!sApp->isNetworkConnectedAndAccessible())
+                if (!sApp->isNetworkAccessible())
                 {
                     CLogMessage(this).error("No weather download since network not accessible");
                     return;