project: Fix and reinforce tarball export

With GES Assets, we need to use source.get_id() instead of get_uri()

The rest of this commit just hardens the code to be a bit more fault-tolerant.
project: Prefix file extensions of exported tarballs with the GES formatter ext

This is in preparation for a feature in GES that could allow loading tarballs,
and to make those files easier to recognize for applications and humans as well.