nmtui: fix sensitivity of "Add" button in address lists (rh #1131434)

When you first click "Add" in an address list, it adds a new field,
and the "Add" button becomes insensitive (because the new field is
empty, and thus not valid). But if you then clicked "Remove", "Add"
stayed insensitive. Fix that.

clients/tui/nmt-widget-list.c

@@ -243,7 +243,8 @@ ensure_widgets (NmtWidgetList *list)
 		                                                G_BINDING_SYNC_CREATE);
 		g_object_add_weak_pointer (G_OBJECT (priv->add_sensitivity),
 		                           (gpointer *)&priv->add_sensitivity);
-	}
+	} else
+		nmt_newt_component_set_sensitive (NMT_NEWT_COMPONENT (priv->add_button), TRUE);
 }
 
 static void