Add Dropper BlockState. Adds BUKKIT-3750

By: Xephi <xephirot59@gmail.com>

paper-api/src/main/java/org/bukkit/block/Dropper.java

@@ -0,0 +1,25 @@
+package org.bukkit.block;
+
+import org.bukkit.inventory.InventoryHolder;
+
+/**
+ * Represents a dropper.
+ */
+public interface Dropper extends BlockState, InventoryHolder {
+    /**
+     * Tries to drop a randomly selected item from the Dropper's inventory,
+     * following the normal behavior of a Dropper.
+     * <p />
+     * Normal behavior of a Dropper is as follows:
+     * <p />
+     * If the block that the Dropper is facing is an InventoryHolder or
+     * ContainerBlock the randomly selected ItemStack is placed within that 
+     * Inventory in the first slot that's available, starting with 0 and
+     * counting up.  If the inventory is full, nothing happens.
+     * <p />
+     * If the block that the Dropper is facing is not an InventoryHolder or
+     * ContainerBlock, the randomly selected ItemStack is dropped on
+     * the ground in the form of an {@link org.bukkit.entity.Item Item}.
+     */
+     public void drop();
+}