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/Palette.java

@@ -89,13 +89,13 @@ public class Palette
             rgb8[i] = encodeRGB8( colours[i] );
         }
 
-        nbt.setIntArray( "term_palette", rgb8 );
+        nbt.putIntArray( "term_palette", rgb8 );
         return nbt;
     }
 
     public void readFromNBT( NBTTagCompound nbt )
     {
-        if( !nbt.hasKey( "term_palette" ) ) return;
+        if( !nbt.contains( "term_palette" ) ) return;
         int[] rgb8 = nbt.getIntArray( "term_palette" );
 
         if( rgb8.length != colours.length ) return;