Support adding Bitcoin, Ethereum and Stellar wallets through the UI

[bjorn]

Raccoin supports importing Bitcoin, Ethereum and Stellar wallets directly from the blockchain. However, it is currently not possible to add these wallets using the UI. Instead, it requires manually adding them to the portfolio JSON file as documented at https://raccoin.org/reference/#blockchains.

It should really be possible to add these through the UI on the Wallets page, either using some kind of pop-up or through additional buttons.