Harden FTPManager::download against packet loss

Component Information FTP download support

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

- Also corrected system/component id usage
- Switched to new Qt5 connect api

Added:
- sequence numbers
- better filtering for image encapsulated data messages versus FTP
- directory list now contains file sizes
- open command now returns file size in data

Also changed the downloaded file contents to not include the file
length in the first byte. This makes the code a bit less confusing.

Also modified protocol to better support eof’s on list and read
commands.