Decouple auto-backup to bookmarks and file

[AlirezaDastyar]

Is your feature request related to a problem? Please describe.
Yes, auto-backup directory is in the download folder and as it has come up in form of issues (many).
i.e. :

• Backups should not be saved to downloads #778
• If Simple Tab Groups makes 'STG-backups,' please save in Mozilla profile rather than user's home #284
• Backup location #693

Not only it's not a good privacy practice to store browsing history in plain text inside a public directory such as Downloads, Also it doesn't make sense to see a backup file in Downloads directory.

Describe the solution you'd like
As it has been cleared (in other issues) that the alternative is to turn-off auto-backup.
And since backup to bookmarks is coupled with backup to file I'd suggest add an option to separate the two.
It's better than alternative (turning off the auto-backup).

Describe alternatives you've considered
I don't know if it's a limitation or a choice but if the backup directory name could have a dot at the beginning it will be hidden in unix-like systems, currently dots at beginning are discarded.
But it's just for looks, I'd my first suggestion better.

Additional context
I did a proof of concept here in this commit.
I can make PR if you are interested.

Cheers.