Add new wallet type: Hardware Wallet

[brusherru]

Such a wallet does not need to have a mnemonic inside. It will use only imported keys.

The problem here is that GUI should take it into account and:

1. implement different wallet creating workflow (e.g. require to import any key before actually creating the wallet),
2. restrict creating accounts from mnemonics,
3. allow to generate mnemonics later (aka convert into common wallet).