Add EntityMountEvent and EntityDismount Event

Adapted from Spigot commit ab1e1a2a5a.

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

paper-api/src/main/java/org/bukkit/event/entity/EntityDismountEvent.java

@@ -0,0 +1,52 @@
+package org.bukkit.event.entity;
+
+import org.bukkit.entity.Entity;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Called when an entity stops riding another entity.
+ */
+public class EntityDismountEvent extends EntityEvent implements Cancellable {
+
+    private static final HandlerList handlers = new HandlerList();
+    private boolean cancelled;
+    private final Entity dismounted;
+
+    public EntityDismountEvent(@NotNull Entity what, @NotNull Entity dismounted) {
+        super(what);
+        this.dismounted = dismounted;
+    }
+
+    /**
+     * Gets the entity which will no longer be ridden.
+     *
+     * @return dismounted entity
+     */
+    @NotNull
+    public Entity getDismounted() {
+        return dismounted;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    @NotNull
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    @NotNull
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}

paper-api/src/main/java/org/bukkit/event/entity/EntityMountEvent.java

@@ -0,0 +1,52 @@
+package org.bukkit.event.entity;
+
+import org.bukkit.entity.Entity;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Called when an entity attempts to ride another entity.
+ */
+public class EntityMountEvent extends EntityEvent implements Cancellable {
+
+    private static final HandlerList handlers = new HandlerList();
+    private boolean cancelled;
+    private final Entity mount;
+
+    public EntityMountEvent(@NotNull Entity what, @NotNull Entity mount) {
+        super(what);
+        this.mount = mount;
+    }
+
+    /**
+     * Gets the entity which will be ridden.
+     *
+     * @return mounted entity
+     */
+    @NotNull
+    public Entity getMount() {
+        return mount;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    @NotNull
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    @NotNull
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}