Change the file transfer protocol

Created by: agraubert

I don't like how the file transfer will immediately transfer a file to your computer before asking. I have two ideas for this solution:

1. Add an additional cipher to the recipient's end. The downloaded file is ciphered as it comes in, and is only deciphered when it's saved. That way the file as sent never exists on your computer until you ask for it.
2. Split file transfer into 2 parts. The sender uses .sendfile() which should place the filepath into a queue of files to send, and immediately send a command to the recipient to request a file transfer. The recipient then uses .savefile() which sends a command back to the sender, which initiates the actual transfer.