Rework mod enumeration / caching.

- Replaced ModMetadata.AllMods with Game.Mods.
- Store / reference mod Manifest instead of ModMetadata.
- Removes engine dependency on ModContent class.

OpenRA.Mods.Common/Widgets/Logic/MainMenuLogic.cs

@@ -90,7 +90,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 				// so we can't do this inside the input handler.
 				Game.RunAfterTick(() =>
 				{
-					Game.Settings.Game.PreviousMod = modData.Manifest.Mod.Id;
+					Game.Settings.Game.PreviousMod = modData.Manifest.Id;
 					Game.InitializeMod("modchooser", null);
 				});
 			};
@@ -283,8 +283,8 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 					{
 						// Send the mod and engine version to support version-filtered news (update prompts)
 						var newsURL = Game.Settings.Game.NewsUrl + "?version={0}&mod={1}&modversion={2}".F(
-							Uri.EscapeUriString(ModMetadata.AllMods["modchooser"].Version),
-							Uri.EscapeUriString(Game.ModData.Manifest.Mod.Id),
+							Uri.EscapeUriString(Game.Mods["modchooser"].Mod.Version),
+							Uri.EscapeUriString(Game.ModData.Manifest.Id),
 							Uri.EscapeUriString(Game.ModData.Manifest.Mod.Version));
 
 						// Append system profile data if the player has opted in