PDF-handling code from `ZoteroPane.viewAttachment()` predates
`Zotero.OpenPDF` and didn't handle page numbers for external PDF
readers, so when called from "Show on Page" in a note (via
`ZoteroPane.viewPDF()`) it just launched the file itself in the
specified handler or via the OS.
For now, change to use `Zotero.OpenPDF.openToPage()` when there's a page
number. Later we should use `Zotero.OpenPDF` for everything, but that
code is more complicated than just using `launchFile()`, so we'll need
to do some testing (particularly on Linux) to see what's best.
(Note that "Show on Page" with an external reader doesn't make a ton of
sense, since you can't actually see the annotation, but opening the
right page is better than nothing.)
Fixes#2612