Separated from #7169.
We still want to get rid of the current SentboxWatch implementation. Maybe leave SentboxWatch as an advanced option, it may be useful, but instead of creating additional IMAP connection just scan it in scan_folders(). So messages from "Sent" may arrive one minute later, shouldn't be a big problem. And enable it by default except for Gmail because it duplicates all sent messages in "Sent" and it's just useless to scan this folder, we only prefetch the same messages again and spend time and battery.

SentBoxWatch should be enabled by default then to preserve the current behavior: if it's disabled, scan_folders() scans it.