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.

doc/events/file_transfer.md

@@ -0,0 +1,41 @@
+---
+module: [kind=event] file_transfer
+---
+
+The @{file_transfer} event is queued when a user drags-and-drops a file on an open computer.
+
+This event contains a single argument, that in turn has a single method @{TransferredFiles.getFiles|getFiles}. This
+returns the list of files that are being transferred. Each file is a @{fs.BinaryReadHandle|binary file handle} with an
+additional @{TransferredFile.getName|getName} method.
+
+## Return values
+1. @{string}: The event name
+2. @{TransferredFiles}: The list of transferred files.
+
+## Example
+Waits for a user to drop files on top of the computer, then prints the list of files and the size of each file.
+
+```lua
+local _, files = os.pullEvent("file_transfer")
+for _, file in ipairs(files.getFiles()) do
+  -- Seek to the end of the file to get its size, then go back to the beginning.
+  local size = file.seek("end")
+  file.seek("set", 0)
+
+  print(file.getName() .. " " .. file.getSize())
+end
+```
+
+## Example
+Save each transferred file to the computer's storage.
+
+```lua
+local _, files = os.pullEvent("file_transfer")
+for _, file in ipairs(files.getFiles()) do
+  local handle = fs.open(file.getName(), "wb")
+  handle.write(file.readAll())
+
+  handle.close()
+  file.close()
+end
+```