tracker-sandbox: Setup xdg-user-dirs

All of the XDG user-dirs will be set to the content directory's
path, so that applications such as gnome-documents can find the files
in the expected directories.

https://bugzilla.gnome.org/show_bug.cgi?id=735636
tracker-extract: Show where parsing errors happen in EPubs

Error extracting EPUB contents (OEBPS/Text/info.xhtml): Error on line 59: Entity name 'copy' is not known
is better than:
Error extracting EPUB contents: Error on line 59: Entity name 'copy' is not known

https://bugzilla.gnome.org/show_bug.cgi?id=735645