Emit “applied” once for each applied update and once for each client.

[Jonas G. Drange]

Applied cannot be a signal that is sent to the client only if the client was connected to s-i at time of the event. A client needs to know if an update was applied, and what build number that update had.

Reproduce:
Apply system update

What happens:
No way to know if update was applied or not, besides reading current build number (might have changed)

What should happen:
For each new client that connects to the <backend>, a signal should be emitted for that client.

So for some client that's really out of the loop (updates 301, 302 and 303 was applied after the last time it connected to the <backend>), it will receive the following signals when it connects to the <backend>:

    applied(301)
    applied(302)
    applied(303)