Apply $DefaultQueue's display/value callbacks to $RTIR_DefaultQueue

With the callbacks, RTIR_DefaultQueue now shows queue name on system
configuration page and the default value works on config edit page too.

Author: sunnavy

lib/RT/IR/Config.pm

@@ -161,7 +161,9 @@ sub Init {
                         }
                         return $ret;
                     },
-                }
+                    ValueCallback => RT->Config->Meta('DefaultQueue')->{WidgetArguments}{ValueCallback},
+                },
+                DisplayCallback => RT->Config->Meta('DefaultQueue')->{DisplayCallback},
             },
             RTIR_StrictConstituencyLinking => {
                 Type   => 'SCALAR',