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/Installation/ModContentPromptLogic.cs

@@ -21,10 +21,10 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 	public class ModContentPromptLogic : ChromeLogic
 	{
 		[TranslationReference]
-		const string Continue = "continue";
+		const string Continue = "button-continue";
 
 		[TranslationReference]
-		const string Quit = "quit";
+		const string Quit = "button-quit";
 
 		readonly ModContent content;
 		bool requiredContentInstalled;