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

mods/ra/languages/rules/en.ftl

@@ -1,23 +1,25 @@
 ## Player
-infantry-only = Infantry Only
-low = Low
-medium = Medium
-no-superweapons = No Superweapons
-unrestricted = Unrestricted
+options-tech-level =
+    .infantry-only = Infantry Only
+    .low = Low
+    .medium = Medium
+    .no-superweapons = No Superweapons
+    .unrestricted = Unrestricted
 
-kill-bounties =
+checkbox-kill-bounties =
     .label = Kill Bounties
     .description = Players receive cash bonuses when killing enemy units
 
-redeployable-mcvs =
+checkbox-redeployable-mcvs =
     .label = Redeployable MCVs
     .description = Allow undeploying Construction Yard
 
-reusable-engineers =
+checkbox-reusable-engineers =
     .label = Reusable Engineers
     .description = Engineers remain on the battlefield after capturing a structure
 
 ## World
-mcv-only = MCV Only
-light-support = Light Support
-heavy-support = Heavy Support
+options-starting-units =
+    .mcv-only = MCV Only
+    .light-support = Light Support
+    .heavy-support = Heavy Support