Allow importing/migrating wallets from wallet.db files from different locations

[dbadol]

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Enhancement description After installing wallet 7.5 for the first time, the "start manual migration" option allows to select an existing wallet.db to be migrated (from the default location, or from any path selected by the user). However, this possibility is not available anymore for the following accounts that the user might create (they can only create a new seed, restore a seed or use an hardware wallet).

Expected behaviour The "Add account"/"Restore wallet" option from the main screen should allow to:

1. Restore from seed (current behavior)
2. Restore from file (migrate from an existing wallet.db file).

Platform and build Windows, Mac, Linux Beam Wallet 7.5

Screenshot The needed GUI for option 2 already exist in the normal migration process at first launch ("start manual migration").