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: it's not a Qt class so probably shouldn't have the "Q" prefix (it's nice to keep it that way since then you can tell what classes are defined by Mixxx and Qt at a glance).