Qt: Add GUI feedback and control of network activity state.

Add getNetworkActive()/setNetworkActive() method to client model.
Send network active status through NotifyNetworkActiveChanged.
Indicate in tool tip of gui status bar network indicator whether network activity is disabled.
Indicate in debug window whether network activity is disabled and add button to allow user to toggle network activity state.

src/net.cpp

@@ -2050,6 +2050,8 @@ void CConnman::SetNetworkActive(bool active)
     } else {
         fNetworkActive = true;
     }
+
+    uiInterface.NotifyNetworkActiveChanged(fNetworkActive);
 }
 
 CConnman::CConnman(uint64_t nSeed0In, uint64_t nSeed1In) : nSeed0(nSeed0In), nSeed1(nSeed1In)