Support multi-account functionality.

[nuttycom]

Is your feature request related to a problem? Please describe.

It would be very useful to be able to have the ability to have multiple "view-only" accounts in addition to the main spending account of the wallet.

Describe the solution you'd like

Make it possible to import full and incoming viewing keys to an existing wallet with spending capability. When I import a viewing key, I should be prompted to enter a name for the account. The main screen should include an account switcher that shows these names.

It should be possible mark a specific account as "sensitive" such that it requires both a PIN and biometric auth to display the balance and history for that account. Cold storage wallets will usually store more value than hot wallets, and so the balance information should be treated as more sensitive.