Add ScriptLobbyDropdown trait and lua API.

OpenRA.Mods.Common/Scripting/Global/MapGlobal.cs

@@ -107,6 +107,18 @@ namespace OpenRA.Mods.Common.Scripting
 		[Desc("Returns the difficulty selected by the player before starting the mission.")]
 		public string Difficulty { get { return Context.World.LobbyInfo.GlobalSettings.Difficulty; } }
 
+		[Desc("Returns the value of a `ScriptLobbyDropdown` selected in the game lobby.")]
+		public LuaValue LobbyOption(string id)
+		{
+			var option = Context.World.WorldActor.TraitsImplementing<ScriptLobbyDropdown>()
+				.FirstOrDefault(sld => sld.Info.ID == id);
+
+			if (option == null)
+				throw new YamlException("A ScriptLobbyDropdown with ID `" + id + "` was not found.");
+
+			return option.Value;
+		}
+
 		[Desc("Returns a table of all the actors that were specified in the map file.")]
 		public Actor[] NamedActors { get { return sma.Actors.Values.ToArray(); } }