Add interface for spawning FallingBlocks and correctly spawn a FallingBlock with the spawn(Location, FallingBlock.class) method. Adds BUKKIT-2282

Also add FallingBlock and methods.

Deprecated FallingSand to emphasize FallingBlock.

By: feildmaster <admin@feildmaster.com>

paper-api/src/main/java/org/bukkit/entity/FallingSand.java

@@ -2,5 +2,8 @@ package org.bukkit.entity;
 
 /**
  * Represents a falling block.
+ *
+ * @deprecated See {@link FallingBlock}
  */
-public interface FallingSand extends Entity {}
+@Deprecated
+public interface FallingSand extends FallingBlock {}