Restore from a different backup without affecting current backup settings

[t-m-w]

Alternative title: One-time restore from a different backup

Use case: A user has a device already set up with Seedvault, and they have another backup set from another device that they want to restore data from. Perhaps it's an older backup that failed to restore in the past due to a glitch that has since been fixed, and they want to grab some apps or files from it.

If the backup set can't be decrypted with the same recovery code, Seedvault could ask for it (but not alter anything it has saved, so as not to interfere with the current backup set).

Now that #723 is merged, we already have a Restore button to initiate restore from the same backup set.

Not sure if this feature is too complicated to be feasible.

[grote]

Current work-around: clear app data, choose restore on first start from other location.

Then to move back to previous location: clear app data again then choose restore from previous location, but skip actual restore.