Update CC: Tweaked to 1.13

Look, I originally had this split into several commits, but lots of
other cleanups got mixed in. I then backported some of the cleanups to
1.12, did other tidy ups there, and eventually the web of merges was
unreadable.

Yes, this is a horrible mess, but it's still nicer than it was. Anyway,
changes:

 - Flatten everything. For instance, there are now three instances of
   BlockComputer, two BlockTurtle, ItemPocketComputer. There's also no
   more BlockPeripheral (thank heavens) - there's separate block classes
   for each peripheral type.

 - Remove pretty much all legacy code. As we're breaking world
   compatibility anyway, we can remove all the code to load worlds from
   1.4 days.
 - The command system is largely rewriten to take advantage of 1.13's
   new system. It's very fancy!

 - WidgetTerminal now uses Minecraft's "GUI listener" system.

 - BREAKING CHANGE: All the codes in keys.lua are different, due to the
   move to LWJGL 3. Hopefully this won't have too much of an impact.

   I don't want to map to the old key codes on the Java side, as there
   always ends up being small but slight inconsistencies. IMO it's
   better to make a clean break - people should be using keys rather
   than hard coding the constants anyway.

 - commands.list now allows fetching sub-commands. The ROM has already
   been updated to allow fancy usage such as commands.time.set("noon").

 - Turtles, modems and cables can be waterlogged.

src/main/java/dan200/computercraft/shared/util/TickScheduler.java

@@ -9,12 +9,12 @@ package dan200.computercraft.shared.util;
 import com.google.common.collect.MapMaker;
 import dan200.computercraft.ComputerCraft;
 import dan200.computercraft.shared.common.TileGeneric;
-import net.minecraft.block.Block;
 import net.minecraft.tileentity.TileEntity;
 import net.minecraft.util.math.BlockPos;
+import net.minecraft.world.ITickList;
 import net.minecraft.world.World;
+import net.minecraftforge.eventbus.api.SubscribeEvent;
 import net.minecraftforge.fml.common.Mod;
-import net.minecraftforge.fml.common.eventhandler.SubscribeEvent;
 import net.minecraftforge.fml.common.gameevent.TickEvent;
 
 import java.util.Collections;
@@ -22,7 +22,7 @@ import java.util.Iterator;
 import java.util.Set;
 
 /**
- * A thread-safe version of {@link World#scheduleUpdate(BlockPos, Block, int)}.
+ * A thread-safe version of {@link ITickList#scheduleTick(BlockPos, Object, int)}.
  *
  * We use this when modems and other peripherals change a block in a different thread.
  */
@@ -61,7 +61,7 @@ public final class TickScheduler
 
             if( world != null && pos != null && world.isBlockLoaded( pos ) && world.getTileEntity( pos ) == tile )
             {
-                world.scheduleUpdate( pos, tile.getBlockType(), 0 );
+                world.getPendingBlockTicks().scheduleTick( pos, tile.getBlockState().getBlock(), 0 );
             }
         }
     }