The relative order of the on-file-load logic matters, so signals don't
work very well for it. Doing it all explicitly in Project fixes a bunch
of inconsistent issues related to restoring saved state.
Closes#1759.
Remove it from the script info section and put it in its own section
that isn't tracked by undo and make it not stringly typed. Removes the
need for the gross hack where changes are slipped in just before saving
to circumvent the undo system, cuts down on the uses of string literals
to identify fields, and probably improves performance a little.