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/Installation/ModContentPromptLogic.cs

@@ -17,13 +17,10 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 	public class ModContentPromptLogic : ChromeLogic
 	{
 		[ObjectCreator.UseCtor]
-		public ModContentPromptLogic(Widget widget, string modId, Action continueLoading)
+		public ModContentPromptLogic(Widget widget, Manifest mod, ModContent content, Action continueLoading)
 		{
 			var panel = widget.Get("CONTENT_PROMPT_PANEL");
 
-			var mod = ModMetadata.AllMods[modId];
-			var content = ModMetadata.AllMods[modId].ModContent;
-
 			var headerTemplate = panel.Get<LabelWidget>("HEADER_TEMPLATE");
 			var headerLines = !string.IsNullOrEmpty(content.InstallPromptMessage) ? content.InstallPromptMessage.Replace("\\n", "\n").Split('\n') : new string[0];
 			var headerHeight = 0;
@@ -46,7 +43,8 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 			{
 				Ui.OpenWindow("CONTENT_PANEL", new WidgetArgs
 				{
-					{ "modId", modId },
+					{ "mod", mod },
+					{ "content", content },
 					{ "onCancel", Ui.CloseWindow }
 				});
 			};