[Bleeding] Added Vanish API for hiding players from each other.

Adds Player.hidePlayer, Player.showPlayer, and Player.canSee for managing
what players are hidden from a player. When someone is hidden from a player
the player cannot see them in the user list or /list and they cannot /tell
them so they appear to be completely gone from the server.

By: Travis Watkins <amaranth@ubuntu.com>

paper-api/src/main/java/org/bukkit/command/defaults/ListCommand.java

@@ -19,6 +19,10 @@ public class ListCommand extends VanillaCommand {
         StringBuilder players = new StringBuilder();
 
         for (Player player : Bukkit.getOnlinePlayers()) {
+            // If a player is hidden from the sender don't show them in the list
+            if (sender instanceof Player && !((Player) sender).canSee(player))
+                continue;
+
             if (players.length() > 0) {
                 players.append(", ");
             }

paper-api/src/main/java/org/bukkit/command/defaults/TellCommand.java

@@ -24,7 +24,8 @@ public class TellCommand extends VanillaCommand {
 
         Player player = Bukkit.getPlayerExact(args[0]);
 
-        if (player == null) {
+        // If a player is hidden from the sender pretend they are offline
+        if (player == null || (sender instanceof Player && !((Player) sender).canSee(player))) {
             sender.sendMessage("There's no player by that name online.");
         } else {
             StringBuilder message = new StringBuilder();

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

@@ -490,4 +490,26 @@ public interface Player extends HumanEntity, CommandSender, OfflinePlayer, Plugi
      */
     public void setAllowFlight(boolean flight);
 
+    /**
+     * Hides a player from this player
+     *
+     * @param player Player to hide
+     */
+    public void hidePlayer(Player player);
+
+    /**
+     * Allows this player to see a player that was previously hidden
+     *
+     * @param player Player to show
+     */
+    public void showPlayer(Player player);
+
+    /**
+     * Checks to see if a player has been hidden from this player
+     *
+     * @param player Player to check
+     * @return True if the provided player is not being hidden from this player
+     */
+    public boolean canSee(Player player);
+
 }

paper-api/src/test/java/org/bukkit/plugin/messaging/TestPlayer.java

@@ -609,4 +609,16 @@ public class TestPlayer implements Player {
     public void setBedSpawnLocation(Location location) {
         throw new UnsupportedOperationException("Not supported yet.");
     }
+
+    public void hidePlayer(Player player) {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
+
+    public void showPlayer(Player player) {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
+
+    public boolean canSee(Player player) {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
 }