SPIGOT-1292: BlockState based FlowerPot API.

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

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

@@ -0,0 +1,23 @@
+package org.bukkit.block;
+
+import org.bukkit.material.MaterialData;
+
+public interface FlowerPot extends BlockState {
+
+    /**
+     * Gets the item present in this flower pot.
+     *
+     * @return item present, or null for empty.
+     */
+    MaterialData getContents();
+
+    /**
+     * Sets the item present in this flower pot.
+     *
+     * NOTE: The Vanilla Minecraft client will currently not refresh this until
+     * a block update is triggered.
+     *
+     * @param item new item, or null for empty.
+     */
+    void setContents(MaterialData item);
+}