Showing detailed sync progress (fixes #476)

Added `progressEmitter` class to monitor progress and `DownloadProgress` event. Out of sync dialog shows now detailed info about each file progress in a fancy stacked progress bar.
This should be before the if branch, because the one of the `continue` could be called in the above branch causing the puller to linger forever.
When the file will be retried next time, it will have a new puller which will be registered once again.