Add potential reward to the barrows overlay

- Add potential reward to the barrows side overlay
- Fix checkboxes on the side for brothers slain

All credits to https://github.com/runelite/runelite/pull/1858

runelite-api/src/main/java/net/runelite/api/Varbits.java

@@ -323,6 +323,8 @@ public enum Varbits
 	BARROWS_KILLED_KARIL(460),
 	BARROWS_KILLED_TORAG(461),
 	BARROWS_KILLED_VERAC(462),
+	BARROWS_REWARD_POTENTIAL(463),
+	BARROWS_NPCS_SLAIN(464),
 
 	/**
 	 * Spicy stew ingredients