provide drop-in dbus client as replacement for swupd

[barbieri]

since the idea is to replace swupd without breaking user experience, our dbus client should be a drop-in replacement that talks to the actual swupd via the dbus daemon.

This includes progressive delivery of swupd output, so we can get lots and progress on slow downloads and updates.

It is essential to get this merged into swupd-clienti itself, this way clr could use it by default.

TODO: If the client dies, the server may need to cancel the operation? Is it safe?

[rojkov]

addressed in #7