Allow specifying a maximum amount of past back up multiple backups

A useful feature would be to allow the user to set a maximum amount of backups to keep when using "back up multiple" so that it can be set in a profile and run e.g. every day without fear that it would cause too much storage to be used.

AM would then, on backup, decide if the next backup is going to be "too many" and if so, delete the oldest one from the app's backups folder.

Bonus feature: freezing backups — a single backup could be "frozen" or "locked" so that it is never deleted by AM.