The macOS window restoration system is no longer used except
for migrating users from a previous version to this one. We
save state in a SQLite database under Application Support
now. The benefit is that we can avoid saving giant
scrollback histories that have not changed, or at least only
saving the very last part of it when the beginning is
unchanged. Hopefully it should be more reliable, as well.

The previous implementation that works kind of like Apple's
is still here in case of emergency but it can be removed
after this is in a safe place.

For all the gory details, see the delta_encoding branch.

Fixes these issues:
#6632
#7909
#7278
#6189
#5164
#4667
#8990