[backups] Automatic backups basics

- allow to do automatic backups when keypool gets refilled
- by default backups are turned off
- by default only encrypted wallets are allowed for backup (configurable)

This could be the basic for later adding backup possibilities for Bitcoin-Qt including visual elements for latest and amount of backups as also validity-check-display of backup files.
I would say it's needed. When calling `GetAvailableBackups` you then can expect a empty hash-map even when there was a error.