Notify user when detecting multiple synchronization account on the same folder

[mgallien]

If the client detects at runtime that multiple clients are syncing the same client local folder, notifies the user via a desktop notification and put a warning in activity list.
That should enable the user to more easily find out without the need to open the settings dialog.
Optionally we could also pause the synchronization until the user solves the issue.
Having multiple clients syncing in the same folder can cause data loss and should be prevented.

[affe42]

Please take into account that you have users with multi-boot systems syncing the same folders from both Windows and Linux side (from NTFS partitions which are accessible to both operating systems). So it might look like multiple clients are syncing the same folder, but effectively they are not, as Windows / Linux are not running at the same time.