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 ideally be moved to the beginning of the `copierRoutine` just as it gets this from the channel, as otherwise it will show one file actually in progress, and a 2nd file with a status bar but not making any progress as it's blocked on `copyChan <- cs`