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/Lobby/LobbyOptionsLogic.cs

@@ -21,7 +21,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 	public class LobbyOptionsLogic : ChromeLogic
 	{
 		[TranslationReference]
-		const string NotAvailable = "not-available";
+		const string NotAvailable = "label-not-available";
 
 		readonly ModData modData;
 		readonly ScrollPanelWidget panel;