When we click the download button in PDF.js, it downloads a blob://qute:...
URL. We can detect that and force a download rather than opening it in PDF.js
again.
Note that what actually happens depends on the Qt version and backend:
QtWebKit (any Qt version):
Downloads always work properly (regardless of Qt version).
QtWebEngine, Qt 5.7.1:
Downloads work.
QtWebEngine, Qt 5.9 - 5.11:
Downloads won't work as we need to tell PDF.js to not use blob: URLs:
https://bugreports.qt.io/browse/QTBUG-70420 - in theory, PDF.js could fall back
to downloading the existing qute:// URL, but it has a whitelist of schemes
which does not include qute://... Since it's not in that whitelist, it just
ends up doing nothing at all.
QtWebEngine, Qt 5.12:
Downloads should hopefully work properly again, as we can register the qute://
scheme with Chromium, which allows us to use blob:// URLs.
This reverts commit 9c731bde85627308fdde4730b0181a014096cb47.
We need to set some PDF.js options, so we can't just use the default viewer
with ?file=...
webelem.javascript_escape got renamed to javascript.string_escape, and a
new javascript.assemble got added to make it easier to call a function
inside a .js file.
