[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/forms/optionsdialog.ui

@@ -242,6 +242,16 @@
          </property>
         </widget>
        </item>
+       <item>
+        <widget class="QCheckBox" name="allowIncoming">
+         <property name="toolTip">
+          <string>Accept connections from outside</string>
+         </property>
+         <property name="text">
+          <string>Allow incoming connections</string>
+         </property>
+        </widget>
+       </item>
        <item>
         <widget class="QCheckBox" name="connectSocks">
          <property name="toolTip">