Handle file transfers inside CraftOS (#1190)

- Add a new file_transfer event. This has the signature
   "file_transfer", TransferredFiles.

   TransferredFiles has a single method getFiles(), which returns a list
   of all transferred files.

 - Add a new "import" program which waits for a file_transfer event and
   writes files to the current directory.

 - If a file_transfer event is not handled (i.e. its getFiles() method
   is not called) within 5 seconds on the client, we display a toast
   informing the user on how to upload a file.

src/main/java/dan200/computercraft/shared/command/CommandComputerCraft.java

@@ -28,6 +28,7 @@ import net.minecraft.entity.player.PlayerInventory;
 import net.minecraft.entity.player.ServerPlayerEntity;
 import net.minecraft.inventory.container.Container;
 import net.minecraft.inventory.container.INamedContainerProvider;
+import net.minecraft.item.ItemStack;
 import net.minecraft.network.play.server.SPlayerPositionLookPacket;
 import net.minecraft.util.math.BlockPos;
 import net.minecraft.util.text.IFormattableTextComponent;
@@ -225,7 +226,7 @@ public final class CommandComputerCraft
                 .executes( context -> {
                     ServerPlayerEntity player = context.getSource().getPlayerOrException();
                     ServerComputer computer = getComputerArgument( context, "computer" );
-                    new ComputerContainerData( computer ).open( player, new INamedContainerProvider()
+                    new ComputerContainerData( computer, ItemStack.EMPTY ).open( player, new INamedContainerProvider()
                     {
                         @Nonnull
                         @Override