[Qt] allow setting listen via GUI

- add DEFAULT_LISTEN in net.h and use in the code (shared
  setting between core and GUI)

Important: This makes it obvious, that we need to re-think the
settings/options handling, as GUI settings are processed before
any parameter-interaction (which is mostly important for network
stuff) in AppInit2()!

src/qt/optionsmodel.cpp

@@ -110,6 +110,11 @@ void OptionsModel::Init()
     if (!SoftSetBoolArg("-upnp", settings.value("fUseUPnP").toBool()))
         addOverriddenOption("-upnp");
 
+    if (!settings.contains("fListen"))
+        settings.setValue("fListen", DEFAULT_LISTEN);
+    if (!SoftSetBoolArg("-listen", settings.value("fListen").toBool()))
+        addOverriddenOption("-listen");
+
     if (!settings.contains("fUseProxy"))
         settings.setValue("fUseProxy", false);
     if (!settings.contains("addrProxy"))
@@ -214,6 +219,8 @@ QVariant OptionsModel::data(const QModelIndex & index, int role) const
             return settings.value("nDatabaseCache");
         case ThreadsScriptVerif:
             return settings.value("nThreadsScriptVerif");
+        case Listen:
+            return settings.value("fListen");
         default:
             return QVariant();
         }
@@ -339,6 +346,12 @@ bool OptionsModel::setData(const QModelIndex & index, const QVariant & value, in
                 setRestartRequired(true);
             }
             break;
+        case Listen:
+            if (settings.value("fListen") != value) {
+                settings.setValue("fListen", value);
+                setRestartRequired(true);
+            }
+            break;
         default:
             break;
         }