Add ability to change player item in hand. Adds BUKKIT-3318

By: Patrick Seidel <psycowithespn1@aim.com>

paper-api/src/main/java/org/bukkit/inventory/PlayerInventory.java

@@ -102,6 +102,16 @@ public interface PlayerInventory extends Inventory {
      */
     public int getHeldItemSlot();
 
+    /**
+     * Set the slot number of the currently held item.
+     * <p>
+     * This validates whether the slot is between 0 and 8 inclusive.
+     *
+     * @param slot The new slot number
+     * @throws IllegalArgumentException Thrown if slot is not between 0 and 8 inclusive
+     */
+    public void setHeldItemSlot(int slot);
+
     /**
      * Clears all matching items from the inventory. Setting either value to -1 will skip it's check,
      *  while setting both to -1 will clear all items in your inventory unconditionally.