This allows Insert -> Image e.g. in Writer to read a PDF file, and
insert the metafile equivalent of the first page into the document.

Currently the original PDF document is lost on import (unlike when
inserting an SVG file).

Change-Id: Ib0472c5d9bd9a1da054353fa3a3a638a1052721e
Reviewed-on: https://gerrit.libreoffice.org/26586Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>