Archive sync skips files already present on target folder

[bitblaster]

Added the same checks present in SyncThreadSyncFile, so that when the Archive sync finds a file already present on the target folder it checks if the file is really the same (via size and date) and if so skips copying it.

[methedeveloper]

+1 for this pull request! I was just searching into the source code how to perform this same task!