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.

Nit: ```model()``` isn't typically inline-able since it's defined in qheaderview.cpp. May as well hoist it out of the loop.