Refactor GameSpeed setting

*Remove internal GameSpeed defaults
 Enforce setting values explicitly all the time
 Require definition of a DefaultSpeed

*Remove Global.Timestep default

*Remove the hacky Timestep/OrderLatency setting via LobbyInfo

*Fix shellmaps ignoring mod-defined gamespeeds

*Make DateTimeGlobal use the MapOptions gamespeed

OpenRA.Game/GameSpeed.cs

@@ -10,26 +10,49 @@
 #endregion
 
 using System.Collections.Generic;
+using System.Linq;
 
 namespace OpenRA
 {
 	public class GameSpeed
 	{
-		public readonly string Name = "Default";
-		public readonly int Timestep = 40;
-		public readonly int OrderLatency = 3;
+		[FieldLoader.Require]
+		public readonly string Name;
+
+		[FieldLoader.Require]
+		public readonly int Timestep;
+
+		[FieldLoader.Require]
+		public readonly int OrderLatency;
 	}
 
 	public class GameSpeeds : IGlobalModData
 	{
+		[FieldLoader.Require]
+		public readonly string DefaultSpeed;
+
 		[FieldLoader.LoadUsing(nameof(LoadSpeeds))]
 		public readonly Dictionary<string, GameSpeed> Speeds;
 
 		static object LoadSpeeds(MiniYaml y)
 		{
 			var ret = new Dictionary<string, GameSpeed>();
-			foreach (var node in y.Nodes)
-				ret.Add(node.Key, FieldLoader.Load<GameSpeed>(node.Value));
+			var speedsNode = y.Nodes.FirstOrDefault(n => n.Key == "Speeds");
+			if (speedsNode == null)
+				throw new YamlException("Error parsing GameSpeeds: Missing Speeds node!");
+
+			foreach (var node in speedsNode.Value.Nodes)
+			{
+				try
+				{
+					ret.Add(node.Key, FieldLoader.Load<GameSpeed>(node.Value));
+				}
+				catch (FieldLoader.MissingFieldsException e)
+				{
+					var label = e.Missing.Length > 1 ? "Required properties missing" : "Required property missing";
+					throw new YamlException("Error parsing GameSpeed {0}: {1}: {2}".F(node.Key, label, e.Missing.JoinWith(", ")));
+				}
+			}
 
 			return ret;
 		}