Updating add-on should not transfer unchanged files

[Benedikt Straub]

https://www.widelands.org/forum/post/38363/

When updating an existing add-on, send only files that have actually changed.

I guess diffing files locally might be complicated since the "original" might not be around, but we can use checksums.
The protocol would then look like this:

• Client sends CMD_SUBMIT with full list of dirs, filenames, and checksums.
• Server sends list of files which need to be replaced:
  • Transfer all files that are not present on the server yet or whose checksum has changed.
  • If a new file has the same checksum as an existing file, we can assume that the file was moved or copied, so we don't need to request it.
  • If multiple files have the same checksum, only one copy needs to be sent.
• Client sends the files requested by the server.
• Server copies unchanged files over into the uploaded directory.