Use xz for distribution

It has better compression.
Implement only the test target, not check

At the moment our tests requires to be run inside a clean
dbus/xvfb session, which is not always available where the
check target is used (for example in distcheck). Also when
running distcheck in buildbot we don't really want to run
tests again, since we just run them in the check step.

We might revisit this in the future but using a non standard
target is the easiest solution for now.
Add missing headers

Fixes distcheck
Release 0.99.0
Avoid auto-import of bundle helpers (#4527)

The new bundle helpers were being auto-loaded with sugar3.bundle due
to their placement in __init__.py. This was causing Gi to be imported,
breaking GTK2 activity launches.

Move the helpers to a dedicated module.