AptURLs don't directly open apturl in Xubuntu

[Ryan Oram]

When I run "exo-open apt://foo" on Xubuntu, by default it opens the
default browser instead of AptURL. This is great if Firefox is your
default browser, as it will then open AptURL on its own. But if your
default browser is Chromium, which uses xdg-open to determine the
default application for URIs, an AptURL will just open another
instance of Chromium and no package will be installed.

I'm looking into making a website for the "Ubuntu AppUpdate" service
and was disappointed to see that Xubuntu doesn't fully support
AptURLs, at least not to the same extent as Ubuntu.

I looked into trying set exo-open manually and it looks like a fiery
undocumented hell, so I was wondering if you guys would know how to
fix this.

Thanks,
Ryan