Add better challenge sorting in AdminGUI's

Now challenges will be sorted by their level order number, their order number, their unique id. All free challenges will always be at the start.

src/main/java/world/bentobox/challenges/ChallengesManager.java

@@ -94,6 +94,45 @@ public class ChallengesManager
     public static final String FREE = "";
 
 
+    // ---------------------------------------------------------------------
+    // Section: Comparators
+    // ---------------------------------------------------------------------
+
+
+    /**
+     * This comparator orders challenges by their level, order and name.
+     */
+    private final Comparator<Challenge> challengeComparator = (o1, o2) -> {
+        if (o1.getLevel().equals(o2.getLevel()))
+        {
+            if (o1.getOrder() == o2.getOrder())
+            {
+                // If orders are equal, sort by unique Id
+                return o1.getUniqueId().compareToIgnoreCase(o2.getUniqueId());
+            }
+            else
+            {
+                // If levels are equal, sort them by order numbers.
+                return Integer.compare(o1.getOrder(), o2.getOrder());
+            }
+        }
+        else
+        {
+            if (o1.getLevel().isEmpty() || o2.getLevel().isEmpty())
+            {
+                // If exist free level challenge, then it should be at the start.
+                return Boolean.compare(o2.getLevel().isEmpty(), o1.getLevel().isEmpty());
+            }
+            else
+            {
+                // Sort by challenges level order numbers
+                return Integer.compare(this.getLevel(o1.getLevel()).getOrder(),
+                    this.getLevel(o2.getLevel()).getOrder());
+            }
+        }
+    };
+
+
     // ---------------------------------------------------------------------
     // Section: Constructor
     // ---------------------------------------------------------------------
@@ -1385,7 +1424,7 @@ public List<String> getAllChallengesNames(@NonNull World world)
     {
         return this.islandWorldManager.getAddon(world).map(gameMode ->
             this.challengeCacheData.values().stream().
-                sorted(Comparator.comparing(Challenge::getOrder)).
+                sorted(this.challengeComparator).
                 filter(challenge -> challenge.matchGameMode(gameMode.getDescription().getName())).
                 map(Challenge::getUniqueId).
                 collect(Collectors.toList())).
@@ -1403,7 +1442,7 @@ public List<Challenge> getAllChallenges(@NonNull World world)
     {
         return this.islandWorldManager.getAddon(world).map(gameMode ->
             this.challengeCacheData.values().stream().
-                sorted(Comparator.comparing(Challenge::getOrder)).
+                sorted(this.challengeComparator).
                 filter(challenge -> challenge.matchGameMode(gameMode.getDescription().getName())).
                 collect(Collectors.toList())).
             orElse(Collections.emptyList());