QtPdf examples: use test.pdf from resources if no file given

On iOS, the native FileDialog doesn't work, sharing doesn't work, and
packaging files along with the application requires manual effort; so
a PDF file in resources seems to be the easiest alternative to make
the examples demo-able.

QPdfDocument wants a file path, because it uses QFile; but we like
to use URLs in Qt Quick.  So it's a bit of an impedance mismatch,
there are several choices about when and where to do the conversion,
and it's hard to say which way is more correct.  This way happens
to work for now.

Also do the rest of the things necessary to get this working on iOS.

Change-Id: Icb8614d5eed2510f101aefba534ef80cf890518f
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>