TrestleBoard 1.3.1

Four independent reviewers went over v1.3.0 as a final check before
calling it production-ready. Nothing they found was a regression
introduced by v1.3.0 -- the two most serious findings were older than
it -- and the seam most likely to fail in the field was verified safe:
a newsletter containing an emblem, sent to somebody still on v1.2.0,
shows them "This newsletter was saved by a newer version of
TrestleBoard. Please update TrestleBoard, then open the file again."
No crash, no lost drawing.

The most important fix, and the oldest:

- Exporting a PDF could destroy last month's. The export wrote straight
  into the file with truncate-on-open, so a full disk or an antivirus
  lock part way through left a broken PDF sitting there under the right
  name with the right icon -- exactly the file that then gets emailed
  to the lodge. It now writes to a temporary file and renames it into
  place, the way every other part of the app already saved, so a
  failure leaves the file that was there completely untouched, and says
  so.

The app no longer says "Done" when you backed out:

- Cancelling the caption or description dialog from a review remedy had
  the review say "Done", letting a real accessibility worry be ticked
  off unfixed. Cancelling "Words for hard news" told a grieving
  committee member the memorial words had gone in. Stopping an import
  had Help say "Done" while the status bar said the import was
  stopped -- two answers to one action. Thirty-odd commands were
  widened so what the app says follows what actually happened.

Read Aloud stops fighting you:

- Editing a sentence while it read to you snapped the page back, took
  the focus, and repeated itself on every keystroke. Editing while
  proofreading is the whole point of reading it back, and it now leaves
  you where you are. It also no longer claims an unrelated sentence is
  "that sentence as it reads now" when the writing it was reading has
  been deleted.

Also fixed: restoring an earlier address book that cannot be read now
refuses instead of quietly saving an empty one over the real thing;
opening a damaged file no longer makes the newsletter you already had
open forget where it lives; a captioned drawing is offered "Change the
caption"; a drawing with no description is asked about in the review;
toggles say when a choice could not be remembered.

Behind the scenes: the internal check meant to stop the app announcing
things it did not do was itself blind to three quarters of the shell,
because a comment containing the words "the record that" was being read
as code. It sees the whole shell now, and everything it found is fixed
rather than excused -- its exception list is empty.

PDFium's licence now ships inside the installer and is shown in Help,
which the project's own rules had required and not had.

Updates should be smaller from here: the release now fetches the
previous version so only the changed parts need downloading, which the
build has claimed to do since the first release and never did. This is
the first release to actually try it.