Display how many bytes of the message were downloaded

[alexei]

For diagnostic/knowing what's going on, it would be nice to have an indicator of how many bytes of the message where actually downloaded from the server (I'm thinking IMAP, but applies to POP as well). Motivation for this ticket is the same as that for #368 : ease of diagnostics and recovery.

The byte count could be added to Message Properties->Structure view somewhere alongside the size in bytes of the mime parts that is already displayed there (and is gotten from the header, iiuc). It would be nice to see that e.g. the message is not displayed because the text mime part is 100 bytes big, but 0 bytes were actually fetched. Then, once #368 is implemented, it would be easy to just force LM to re-fetch that message.