Add Horse API. Adds BUKKIT-4424

API has been added to interface with Horses and to modify their inventories. A new event, HorseJumpEvent, has been added to be fired whenever a horse jumps.

This commit fixes BUKKIT-4393.

src/main/java/org/bukkit/inventory/HorseInventory.java

@@ -0,0 +1,32 @@
+package org.bukkit.inventory;
+
+public interface HorseInventory extends Inventory {
+
+    /**
+     * Gets the item in the horse's saddle slot.
+     *
+     * @return the saddle item
+     */
+    ItemStack getSaddle();
+
+    /**
+     * Gets the item in the horse's armor slot.
+     *
+     * @return the armor item
+     */
+    ItemStack getArmor();
+
+    /**
+     * Sets the item in the horse's saddle slot.
+     *
+     * @param stack the new item
+     */
+    void setSaddle(ItemStack stack);
+
+    /**
+     * Sets the item in the horse's armor slot.
+     *
+     * @param stack the new item
+     */
+    void setArmor(ItemStack stack);
+}