Refactor translation files

- Add prefixes to all message keys to provide context
- Use messages with attributes for some UI elements (dropdowns, dialogs, checkboxes, menus)
- Rename some class fields for consistency with translation keys

OpenRA.Mods.Common/Widgets/Logic/IntroductionPromptLogic.cs

@@ -22,10 +22,10 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 		const int IntroductionVersion = 1;
 
 		[TranslationReference]
-		const string Classic = "classic";
+		const string Classic = "options-control-scheme.classic";
 
 		[TranslationReference]
-		const string Modern = "modern";
+		const string Modern = "options-control-scheme.modern";
 
 		readonly string classic;
 		readonly string modern;