Add system attribute to .sync_*.db* files

Nextcloud desktop client for Microsoft Windows creates/updates these files on every sync

.nextcloudsync.log
.sync_*.db
.sync_*.db-shm
.sync_*.db-wal

To have a "cleaner" look in synced folders, I suggest setting these files attributes to system & hidden instead of only hidden (or at least not remove the system attribute after being manually set by the user).

The purpose of this suggestion is to hide these files while still being able to see hidden files in synced folders