Add update closed event

[BrunoBibboVoicemod]

This is a solution candidate for #127 issue (duplicated #271) where WinSparkle library user can suscribe to a new event emitted when the updater closes (win_sparkle_set_update_closed_callback).

[vslavik]

Sorry, I still (hence the delay in response...) don't get why should this be a separate callback event and not just win_sparkle_set_update_cancelled_callback — it's effectively the same thing, isn't it?

It gets worse, though — I then noticed that there already is win_sparkle_set_update_dismissed_callback, contradicting the above opinion. And that one is never actually called anywhere.

So I think, but am open to arguments, that it should be separate event after all - simply because it was added to the API, however broken, a while back - but that this code should use win_sparkle_set_update_dismissed_callback instead of adding a new one.

[vslavik]

I was wrong about the dismissed callback. The dismissed and cancelled callbacks are a hot mess, but adding yet another confusing one is not a solution, so I decided to close this PR and cleanup the existing callbacks instead.