Expose mod.yaml content to localisation.

Mod metadata, load screens and mod content is all now sourced from ftl files, allowing these items to be translated.

Translations are now initialized as part of ModData creation, as currently they are made available too late for the usage we need here.

The "modcontent" mod learns a new parameter for "Content.TranslationFile" - this allows a mod to provide the path of a translation file to the mod which it can load. This allows mods such as ra, cnc, d2k, ts to own the translations for their ModContent, yet still make them accessible to the modcontent mod.

CheckFluentReference learns to validate all these new fields to ensure translations have been set.

OpenRA.Mods.Common/ModContent.cs

@@ -21,6 +21,7 @@ namespace OpenRA
 	{
 		public class ModPackage
 		{
+			[FluentReference]
 			public readonly string Title;
 			public readonly string Identifier;
 			public readonly string[] TestFiles = Array.Empty<string>();
@@ -100,10 +101,13 @@ namespace OpenRA
 			}
 		}
 
+		[FluentReference]
 		public readonly string InstallPromptMessage;
 		public readonly string QuickDownload;
+		[FluentReference]
 		public readonly string HeaderMessage;
 		public readonly string ContentInstallerMod = "modcontent";
+		public readonly string Translation;
 
 		[FieldLoader.LoadUsing(nameof(LoadPackages))]
 		public readonly Dictionary<string, ModPackage> Packages = new();