Re-add mnemonic names for derivation paths

[laboon]

With Parity Signer, one could have names for derivation paths. So for instance, //polkadot//bill//1 could be displayed as "POLKADOT MAIN" or similar for import into Polkadot-JS App. This also made it easy to see the accounts in a list. Now, however, only the derivation path is listed, and no name. This makes keeping track of many derivation paths difficult.