Add plugin channel events. Addresses BUKKIT-1844

By: Wesley Wolfe <weswolf@aol.com>

paper-api/src/main/java/org/bukkit/event/player/PlayerChannelEvent.java

@@ -0,0 +1,30 @@
+package org.bukkit.event.player;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.HandlerList;
+
+/**
+ * This event is called after a player registers or unregisters a new plugin channel.
+ */
+public abstract class PlayerChannelEvent extends PlayerEvent {
+    private static final HandlerList handlers = new HandlerList();
+    private final String channel;
+
+    public PlayerChannelEvent(final Player player, final String channel) {
+        super(player);
+        this.channel = channel;
+    }
+
+    public final String getChannel() {
+        return channel;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}

paper-api/src/main/java/org/bukkit/event/player/PlayerRegisterChannelEvent.java

@@ -0,0 +1,13 @@
+package org.bukkit.event.player;
+
+import org.bukkit.entity.Player;
+
+/**
+ * This is called immediately after a player registers for a plugin channel.
+ */
+public class PlayerRegisterChannelEvent extends PlayerChannelEvent {
+
+    public PlayerRegisterChannelEvent(final Player player, final String channel) {
+        super(player, channel);
+    }
+}

paper-api/src/main/java/org/bukkit/event/player/PlayerUnregisterChannelEvent.java

@@ -0,0 +1,13 @@
+package org.bukkit.event.player;
+
+import org.bukkit.entity.Player;
+
+/**
+ * This is called immediately after a player unregisters for a plugin channel.
+ */
+public class PlayerUnregisterChannelEvent extends PlayerChannelEvent {
+
+    public PlayerUnregisterChannelEvent(final Player player, final String channel) {
+        super(player, channel);
+    }
+}