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.

Hm, this shouldn't work. This returns the name of the column at the field index that shares the value of the column enum.

What you want is probably: ```return m_columnIndexByName.key(fieldIndex(column));```