Add Scoreboard API and Command. Adds BUKKIT-3776, BUKKIT-3834

The implementation is designed around having both a main scoreboard and
numberous plugin managed scoreboards that can be displayed to specific
players.

Plugin managed scoreboards are active so long as a reference is kept by a
plugin, or it has been registered as a player's active scoreboard. Objects
specific to a scoreboard remain active until unregistered (which remove a
reference to the owning scoreboard), but quickly fail if accessed
post-unregistration.

By: mbax <github@phozop.net>

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

@@ -17,6 +17,7 @@ import org.bukkit.command.CommandSender;
 import org.bukkit.conversations.Conversable;
 import org.bukkit.map.MapView;
 import org.bukkit.plugin.messaging.PluginMessageRecipient;
+import org.bukkit.scoreboard.Scoreboard;
 
 /**
  * Represents a player, connected or not
@@ -626,4 +627,19 @@ public interface Player extends HumanEntity, Conversable, CommandSender, Offline
      * @throws IllegalArgumentException Thrown if the URL is too long.
      */
     public void setTexturePack(String url);
+
+    /**
+     * Gets the Scoreboard displayed to this player
+     *
+     * @return The current scoreboard seen by this player
+     */
+    public Scoreboard getScoreboard();
+
+    /**
+     * Sets the player's visible Scoreboard
+     * Scoreboard must be currently registered or an IllegalArgumentException is thrown
+     *
+     * @param scoreboard New Scoreboard for the player
+     */
+    public void setScoreboard(Scoreboard scoreboard);
 }