epubveri for calibre 0.2.0

Validates the book you have open in calibre's Edit Book with
epubveri — no Java, sub-second, and
reporting epubcheck's own message IDs. It reads your book and reports; it
changes nothing, and the file on disk stays byte-identical.

Install: Edit Book → Preferences → Plugins → Load plugin from file,
choose epubveri_calibre_v0.2.0.zip. Then Plugins → Validate with epubveri,
or the toolbar button.

On first use it downloads the epubveri binary for your platform (~3 MB) and
verifies it against that release's SHA256SUMS.txt. The zip contains no
binary.

The results are a dock, not a window

Asked for by Doitsu — a dockwidget like calibre's own Check Book panel —
with JSWolf adding that it was more than a nitpick. Both right: the dialog
had the correct behaviour and the wrong shape.

Drag it where you want, tab it behind another panel, or close it. calibre
remembers where you put it, because the dock is created early enough in
Edit Book's startup that restoreState finds it — there is nothing of ours
stored anywhere. It stays hidden until you run a validation, the way Live CSS
does, and validating again refills the one panel rather than opening a second.

This is not calibre's own Check Book panel, which stays closed to plugins:
run_checks calls calibre's checkers directly and takes no plugin. A dock of
our own is as close as a plugin can get.

It has an icon

It shipped without one — the toolbar button and the menu entry were text and
nothing else. It is the same mark as the Sigil plugin's, and both are now
generated from one description of the drawing, so the two cannot drift apart.

The summary names the plugin's version

epubveri 0.13.3 (plugin 0.2.0) — VALID …, asked for by Doitsu so that a
pasted line identifies both halves. Most of what has gone wrong in these
plugins so far was the plugin rather than the validator, which makes it the
more useful of the two numbers to have been missing.

The archive says which editor it is for

epubveri_calibre_vX.Y.Z.zip. Both plugins were shipping a file called
epubveri_vX.Y.Z.zip, so a download said nothing about which editor it was
for and the two names would collide once the versions met. PeterT asked
for this.

Verified in calibre

Installed from this archive and run on a real book: the dock appears, docks
and tabs, keeps its place across a restart of Edit Book, activating a row
opens the right file at the right line, and the icon is on the toolbar. 34
tests, but every defect these plugins have had was found by running them in
the editor and none by a test suite, so that is the sentence that matters.

Still macOS only. Nothing here has been run on Windows or Linux.