Documents are downloaded to the private cache directory of the app,
and opened from there. That directory is cleared and created again
every time the application starts up.

Change-Id: I5c05c8ae750b6ced3b419c67d84063e8ee3d84aa
Reviewed-on: https://gerrit.libreoffice.org/16192Reviewed-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Tested-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>
(cherry picked from commit d6f9e415)