Remove "*Proxy" classes

- Move registry code into the various *Registry classes.

   I'm not sure this is any more sensible, but things being registered
   in different places kinda irked me.

 - Everything else (i.e. event listeners) goes in a {Client,Common}Hooks
   class right now. It's not ideal, but I don't think we can split it up
   much.

src/main/java/dan200/computercraft/client/ClientHooks.java

@@ -0,0 +1,39 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2021. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.client;
+
+import dan200.computercraft.ComputerCraft;
+import dan200.computercraft.shared.peripheral.monitor.ClientMonitor;
+import net.minecraftforge.api.distmarker.Dist;
+import net.minecraftforge.client.event.ClientPlayerNetworkEvent;
+import net.minecraftforge.event.world.WorldEvent;
+import net.minecraftforge.eventbus.api.SubscribeEvent;
+import net.minecraftforge.fml.common.Mod;
+
+@Mod.EventBusSubscriber( modid = ComputerCraft.MOD_ID, value = Dist.CLIENT )
+public class ClientHooks
+{
+    @SubscribeEvent
+    public static void onWorldUnload( WorldEvent.Unload event )
+    {
+        if( event.getWorld().isClientSide() )
+        {
+            ClientMonitor.destroyAll();
+        }
+    }
+
+    @SubscribeEvent
+    public static void onLogIn( ClientPlayerNetworkEvent.LoggedInEvent event )
+    {
+        ComputerCraft.clientComputerRegistry.reset();
+    }
+
+    @SubscribeEvent
+    public static void onLogOut( ClientPlayerNetworkEvent.LoggedOutEvent event )
+    {
+        ComputerCraft.clientComputerRegistry.reset();
+    }
+}