[packagekit] Apt backend is back

[Sebastian Heinlein]

Dear fellow developers,

As you have perhaps already seen in the latest commits to master I
decided to revive the Python based APT backend. The backend is ported to
the latest python-apt API.

It features a test suite using python-mox - perhaps this is worth
looking at for the other Python based backend authors. Basically all you
need is a mechanism to set up a chroot, the corrsponding chroot support
in your backend and finally a set of test packages.

Cheers,

Sebastian