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/swiftguistandard/swiftguistdinit.cpp

@@ -153,7 +153,7 @@ void SwiftGuiStd::init()
     this->m_init = true;
     QTimer::singleShot(2500, this, &SwiftGuiStd::ps_verifyDataAvailability);
 
-    if (!CNetworkUtils::hasConnectedInterface())
+    if (!sGui->isNetworkAccessible())
     {
         CLogMessage(this).error("No network interface, software will not work properly");
     }