Add setSeed method to WorldProperties

I think this was originally skipped as it does cause some nasty generation
errors at the edge of currently generated chunks. But some plugins which
are drastically modifying chunk generation may not care about this and
could use this functionality.

src/main/java/org/spongepowered/api/world/storage/WorldProperties.java

@@ -170,6 +170,16 @@ public interface WorldProperties extends DataSerializable {
      */
     long getSeed();
 
+    /**
+     * Sets the seed of this world.
+     * 
+     * <p>Warning: this may cause the edge of currently generated chunks to no
+     * longer align with newly generated chunks.</p>
+     * 
+     * @param seed The new world seed
+     */
+    void setSeed(long seed);
+
     /**
      * Gets the number of ticks which have occurred since the world was created.
      *