[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/net.h

@@ -38,6 +38,8 @@ namespace boost {
 
 /** The maximum number of entries in an 'inv' protocol message */
 static const unsigned int MAX_INV_SZ = 50000;
+/** -listen default */
+static const bool DEFAULT_LISTEN = true;
 /** -upnp default */
 #ifdef USE_UPNP
 static const bool DEFAULT_UPNP = USE_UPNP;