Fit the page on startup, name the bundle, show the selection

Three things a first session trips over.

The editor opened at an arbitrary zoom. show_page does fit the page, but
it runs before the window has been laid out, when the viewport is still
its default size; redo it once when the real size arrives.

The bundle was named after the PDF, which is whatever the download was
called. Take the song's title instead — unsafe characters dropped, then
whitespace collapsed to dashes. Accented letters stay, since ä and ö are
not a filesystem's problem, but a leading dot would hide the file.

And the selected slice was drawn as an outline whose top and bottom edges
run under the cut lines painted over them, leaving two thin verticals in
the margins and no way to tell what was selected. Wash the slice, as the
discard and engraved states already do, and keep the outline for the trim
anomalies it exists to show.

docs/guide.md

@@ -78,7 +78,8 @@ and "Andante" cannot.
 
 ## Export
 
-**Export bundle…**, choose where the `.zip` goes, done. Inside are the slice
+**Export bundle…**, choose where the `.zip` goes, done. It is named after the
+song's title — *Bicycle Race* becomes `Bicycle-Race.zip`. Inside are the slice
 images in order, their markers, the song metadata, and the original PDF as the
 archive copy. That zip is the whole interface to noteman; hand it over and open
 it there.