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/HotkeyDefinition.cs

@@ -17,9 +17,15 @@ namespace OpenRA
 	{
 		public readonly string Name;
 		public readonly Hotkey Default = Hotkey.Invalid;
+
+		[FluentReference]
 		public readonly string Description = "";
+
 		public readonly HashSet<string> Types = new();
+
+		[FluentReference]
 		public readonly HashSet<string> Contexts = new();
+
 		public readonly bool Readonly = false;
 		public bool HasDuplicates { get; internal set; }