Move ModContent out of the engine.

OpenRA.Game/Game.cs

@@ -364,13 +364,8 @@ namespace OpenRA
 			using (new PerfTimer("LoadMaps"))
 				ModData.MapCache.LoadMaps();
 
-			var content = ModData.Manifest.Get<ModContent>();
-			var isModContentInstalled = content.Packages
-				.Where(p => p.Value.Required)
-				.All(p => p.Value.TestFiles.All(f => File.Exists(Platform.ResolvePath(f))));
-
 			// Mod assets are missing!
-			if (!isModContentInstalled)
+			if (!ModData.LoadScreen.RequiredContentIsInstalled())
 			{
 				InitializeMod("modchooser", new Arguments());
 				return;