Add the ability to import an existing HD wallet using an extended
private key (xprv) instead of a mnemonic phrase.
- New 'xprv' wallet type with full HD derivation and address scanning
- New importXprv view with password encryption
- Updated getSignerForAddress to handle xprv wallet type
- Added xprv link to the add-wallet view
- Allow adding derived addresses for xprv wallets
Closes#20
