Report which apps require update in DownloadStarted event

[detsch]

This is what the DonwloadStarted event details look like for an "apps sync" update operation:

Syncing Active Target Apps
- shellhttpd2: missing installation, to be re-installed

(updated the above example after the latest force push)

I've started by only adding the app name, but decided to go a little further by adding the specific reason as well, so I created a new patch on top of the original implementation.

[mike-sul]

@detsch This is the nice idea, thanks for doing it.

[detsch]

I've updated the internal structure used to store the apps and reasons from vector to map, so the format is fully applied when building the reason string, not concatenating app name and reason before that.

[detsch]

@mike-sul I've taken a look into adding apps to update + reasons to the "update to target N+1" event, but it is not as trivial as this PR. We currently send the DownloadStarted event before knowing which apps need to be updated, and the code structure there is more complex, a lot more changes would be needed. I'll keep an eye on it and if I visualize some change that makes sense in that direction, I may try something. But it is not worth to deal with it in this PR.

[mike-sul]

I'll keep an eye on it and if I visualize some change that makes sense in that direction, I may try something. But it is not worth to deal with it in this PR.

No need to worry about it. It is not so important for the regular update case (N to N+1 update).