Delete previous chat messages of the same type

This uses a custom ComputerCraft packet to send chat messages to the
client. When received, we delete all messages of the same category
before sending the new ones.

This avoids cluttering the chat with near-identical messages, and helps
make working with the "individual dump" command easier, as the previous
computer's dump output is deleted.

Also change the max height of the TextTable to 18, so it fits within
Minecraft's default chat limit.

src/main/java/dan200/computercraft/shared/network/ComputerCraftPacket.java

@@ -33,6 +33,7 @@ public class ComputerCraftPacket
     public static final byte ComputerTerminalChanged = 8;
     public static final byte ComputerDeleted = 9;
     public static final byte PlayRecord = 10;
+    public static final byte PostChat = 11;
 
     // Packet class
     public byte m_packetType;