New wallet type: SLIP-44 Multi-Currency Wallet

[niahmiah]

We need to provide a better experience for multi-currency users.

By following the BIP-44 spec, expanded upon by the Satoshi Labs team in https://github.com/satoshilabs/slips/blob/master/slip-0044.md, we can use a single Key to provide wallets for all of the currencies we currently support, and possibilities for future expansion.

The current backend architecture would not need to change, but the wallet UI would have to register each branch of the HD key that is used for each currency with the respective backend services.

So, with a purely UI change, we can provide the user with a single backup/restore process for multi currency wallets. The user can simply flip a switch to enable a currency, instead of going through the key generation process again.

[niahmiah]

related, future scope: https://github.com/satoshilabs/slips/blob/master/slip-0048.md