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/blackcore/threadedreader.cpp

@@ -71,7 +71,7 @@ namespace BlackCore
 
     bool CThreadedReader::isNetworkConnectedAndAccessible() const
     {
-        return sApp->isNetworkConnectedAndAccessible();
+        return sApp->isNetworkAccessible();
     }
 
     void CThreadedReader::gracefulShutdown()