Implement CauldronLevelChangeEvent.

Idea for implementation via matiki1231 on IRC.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/event/block/CauldronLevelChangeEvent.java

@@ -0,0 +1,110 @@
+package org.bukkit.event.block;
+
+import com.google.common.base.Preconditions;
+import org.bukkit.block.Block;
+import org.bukkit.entity.Entity;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+
+public class CauldronLevelChangeEvent extends BlockEvent implements Cancellable {
+
+    private static final HandlerList handlers = new HandlerList();
+    private boolean cancelled;
+    //
+    private final Entity entity;
+    private final ChangeReason reason;
+    private final int oldLevel;
+    private int newLevel;
+
+    public CauldronLevelChangeEvent(Block block, Entity entity, ChangeReason reason, int oldLevel, int newLevel) {
+        super(block);
+        this.entity = entity;
+        this.reason = reason;
+        this.oldLevel = oldLevel;
+        this.newLevel = newLevel;
+    }
+
+    /**
+     * Get entity which did this. May be null.
+     *
+     * @return acting entity
+     */
+    public Entity getEntity() {
+        return entity;
+    }
+
+    public ChangeReason getReason() {
+        return reason;
+    }
+
+    public int getOldLevel() {
+        return oldLevel;
+    }
+
+    public int getNewLevel() {
+        return newLevel;
+    }
+
+    public void setNewLevel(int newLevel) {
+        Preconditions.checkArgument(0 <= newLevel && newLevel <= 3, "Cauldron level out of bounds 0 <= %s <= 3", newLevel);
+        this.newLevel = newLevel;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancelled) {
+        this.cancelled = cancelled;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    public enum ChangeReason {
+        /**
+         * Player emptying the cauldron by filling their bucket.
+         */
+        BUCKET_FILL,
+        /**
+         * Player filling the cauldron by emptying their bucket.
+         */
+        BUCKET_EMPTY,
+        /**
+         * Player emptying the cauldron by filling their bottle.
+         */
+        BOTTLE_FILL,
+        /**
+         * Player filling the cauldron by emptying their bottle.
+         */
+        BOTTLE_EMPTY,
+        /**
+         * Player cleaning their banner.
+         */
+        BANNER_WASH,
+        /**
+         * Player cleaning their armor.
+         */
+        ARMOR_WASH,
+        /**
+         * Entity being extinguished.
+         */
+        EXTINGUISH,
+        /**
+         * Evaporating due to biome dryness.
+         */
+        EVAPORATE,
+        /**
+         * Unknown.
+         */
+        UNKNOWN
+    }
+}