Feature: include multisig configuration in backup

[felix-iw]

As a user of multi-sig setups with BitBox hardware wallets, I would like to back up the multi-sig configurations (name, xpubs, etc) together with the seed on SD card so that I can restore the full bitbox configuation easily.

The BitBox already provides good support for multi-sig setups, but sadly the sd card backup does not include the data for the multi-sig wallets.
Even though documentation and app state that the backup only contains the seed, some users may expect the backup to contain all data from the BitBox.

Acceptance criteria:

• the sd card backup must contain the data for all known multi-sig wallets
• the recovery process must also recover the mult-sig configs, if available
• optional: the BitBox app may allow displaying the mult-sig configs (xpubs, derivation path, etc) on the BitBox
• optional: the BitBox may show a warning if the multi-sig config on the BitBox changed after the last backup