We download the summary and send it to the system helper, it verifies
the checksum and applies the changes, with the same polkit permissions
required as for an app update (i.e. typically none).
This allows us to update metadata automatically, without permission
requests.
Drop the intltool dependency that was recently added, and use
upstream gettext and its its features for the same purpose.
Note that polkit currently does not install .its files (I've
sent a patch). Until that is in place, this change has the
effect of installing the untranslated policy file.