Tails Cloner: give more relevant "Backup Instructions" if clone Persistent Storage checked

This MR is based on Fix Tails Cloner bug: button sensitivity or lab... (!1302 - merged)

Adding self.on_target_partitions_changed(None) to the toggle method allows the check button to update the link, without it, checking won't update the link unless source or target change.

I left the Upgrade / Force Reinstall case alone, the link is only changed if the target device is non-Tails and clone Persistent Storage is checked.

This is relatively minor let me know if I should make a UX issue like I did for the new command line option.

Cherry picked from !1292 (closed).