Integrate gtktiledviewer into GNOME-Documents

by pranavk for LibreOffice

Today, GNOME Documents spawns LibreOffice via a rather unreliable unoconv command-line that converts documents to PDF. It is not only unreliable but also the results are not good, for example, spreadsheet rendering results are quite bad. With this project, we would be improving the existing libreofficekit based gtk tiled renderer, which would, then, be a very good replacement of the unreliable unoconv command in gnome-documents.