epubveri for Sigil 0.3.0

• Documented, not changed: positions in an OPF you have edited this session.
  Sigil hands a plugin build_opf() — a rebuild from its own model — rather
  than the file, whenever the OPF is modified, so a line number there belongs
  to the rebuild and can differ from Code View by a line or two. Asked in
  thread 374939. DiapDealer's answer is that the rebuild is what a plugin is
  meant to get, and that the mismatch with Code View should not happen if the
  flush to disk is working — he is checking which of the two it is. The README
  describes it as it behaves today rather than claiming what it is for.
  Nothing else in a book is affected, and an OPF nobody has edited matches
  exactly.

• The order of the findings is a setting now, in the same JSON file as
  the display switches: "sort", one of severity (the default),
  severity-low or document. The words are epubveri's own --sort values
  and the calibre plugin's setting uses exactly the same three, so the two
  can be asked the same question in the same words.

  A setting is the only lever this plugin has, and that is worth stating
  because the calibre one also has clickable columns. Sigil draws its own
  results table: its columns are File / Line / Offset / Message with no
  severity among them, and its sorting is alphabetical — so a user who clicks
  Message gets ERROR, FATAL, INFO, USAGE, WARNING, with a fatal below an
  error. What we choose to send is the only correct severity order available
  here.

  Severest first was already what this plugin did; what changed is that it
  can now be asked for something else, and that each severity group keeps
  the book's order instead of being re-sorted by file name. That is what
  epubveri's --sort severity promises — "within each group the file order is
  unchanged" — and it is the same rule the calibre panel now follows.
  Advisory findings sort last: they never move the verdict.

  Written into the preferences file on first run, like the display switches,
  because the file is the only place it can be changed and therefore has to
  be the place it can be found. A word that is not one of the three opens the
  way the default does.