Add custom library header save/restore code.

Should migrate current 1.12 installations from the old QT binary blob
to the new protobuf format.  This won't help us for users upgrading
from 1.11, but all future upgrades should be much smoother.
includes a small roundtrip test.

I think you want to store the column name not the ColumnCache integer. I can see that being a pain in the future if we want to change the library implementation radically.