Expose hotkeys to localisation.

Allows the Settings > Hotkeys screen to be localised, including hotkey decriptions, groups and contexts.

The hotkey names are exposed to localisation via KeycodeExts. Hotkey modifiers are similarly exposed via ModifersExts.

The Settings > Input screen has a Zoom Modifier dropdown, which shows the localised modifier name.

The --check-yaml utility command is taught to recognise all hotkey translation, so it can validate their usage.

OpenRA.Game/Input/IInputHandler.cs

@@ -10,6 +10,7 @@
 #endregion
 
 using System;
+using System.Collections.Generic;
 
 namespace OpenRA
 {
@@ -61,6 +62,33 @@ namespace OpenRA
 		Meta = 8,
 	}
 
+	public static class ModifiersExts
+	{
+		[FluentReference]
+		public const string Cmd = "keycode-modifier.cmd";
+
+		[FluentReference(Traits.LintDictionaryReference.Values)]
+		public static readonly IReadOnlyDictionary<Modifiers, string> ModifierFluentKeys = new Dictionary<Modifiers, string>()
+		{
+			{ Modifiers.None, "keycode-modifier.none" },
+			{ Modifiers.Shift, "keycode-modifier.shift" },
+			{ Modifiers.Alt, "keycode-modifier.alt" },
+			{ Modifiers.Ctrl, "keycode-modifier.ctrl" },
+			{ Modifiers.Meta, "keycode-modifier.meta" },
+		};
+
+		public static string DisplayString(Modifiers m)
+		{
+			if (m == Modifiers.Meta && Platform.CurrentPlatform == PlatformType.OSX)
+				return FluentProvider.GetString(Cmd);
+
+			if (!ModifierFluentKeys.TryGetValue(m, out var fluentKey))
+				return m.ToString();
+
+			return FluentProvider.GetString(fluentKey);
+		}
+	}
+
 	public enum KeyInputEvent { Down, Up }
 	public struct KeyInput
 	{