[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.h

@@ -42,6 +42,7 @@ public:
         ThreadsScriptVerif,     // int
         DatabaseCache,          // int
         SpendZeroConfChange,    // bool
+        Listen,                 // bool
         OptionIDRowCount,
     };