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.
If we'd change
`output := make(map[string]map[string]pullerProgress)` 
to 
`output := make(map[string]map[string]*pullerProgress)`
then we wouldn't need to dereference the pointer here, and would probably save some RAM (maybe? not sure)