[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/optionsdialog.cpp

@@ -151,6 +151,7 @@ void OptionsDialog::setModel(OptionsModel *model)
     /* Wallet */
     connect(ui->spendZeroConfChange, SIGNAL(clicked(bool)), this, SLOT(showRestartWarning()));
     /* Network */
+    connect(ui->allowIncoming, SIGNAL(clicked(bool)), this, SLOT(showRestartWarning()));
     connect(ui->connectSocks, SIGNAL(clicked(bool)), this, SLOT(showRestartWarning()));
     /* Display */
     connect(ui->lang, SIGNAL(valueChanged()), this, SLOT(showRestartWarning()));
@@ -171,6 +172,7 @@ void OptionsDialog::setMapper()
 
     /* Network */
     mapper->addMapping(ui->mapPortUpnp, OptionsModel::MapPortUPnP);
+    mapper->addMapping(ui->allowIncoming, OptionsModel::Listen);
 
     mapper->addMapping(ui->connectSocks, OptionsModel::ProxyUse);
     mapper->addMapping(ui->proxyIp, OptionsModel::ProxyIP);