Base implementation for new protocol

Step 2 of n in the new protocol (#81)

This implements the first parts of the new protocol described by #118. At this point, it's all new code, and is not used in any way. It's compiled by adding `DEFINES+=PROTOCOL_NEW` to the qmake command, but it won't change behavior at all.

Please review carefully and point out anything that seems fragile.
Mm. You're expecting all the data to be available at the time the header is. That assumption may not always hold true.