XMLUI: Set max-width for item thumbnails in CSS

Now that we're generating larger PDF thumbnails, we need to restrict
the size in CSS.
XMLUI: Set left indent for files list table
dspace.cfg: Enable larger max thumbnail sizes
dspace.cfg: Enable XPDF filter for PDF thumbnails

Requires XPDF to be installed and configured, see section 6.4.2 of
the DSpace manual for more information.