Show fiat estimates

[andywer]

Show fiat estimates in asset details view and send-payment dialog. Ideally in trade dialog and withdrawal dialog, too.

Requires the ability to set a preferred currency in the app settings (EUR, USD, GBP, ...). Use the https://api.satoshipay.io/mainnet/coinmarketcap/v1/cryptocurrency/quotes/latest?convert=EUR&symbol=XLM endpoint to fetch conversion rates, refresh the value once a minute or so.

To Do

• [x] Come up with consistent visual concept where and how to show estimates
• [ ] Add currency preference to app settings (ideally pick initial value based on system locale)
• [ ] Create hook to fetch conversion rate
• [ ] Show estimates in assets details
• [ ] Show estimates in send payment
• [ ] (Nice to have:) Show estimates in trading & withdrawal dialog

[andywer]

Add fiat estimates to balances detail view:

Add fiat estimates to payment, trade & withdrawal view (search for PriceInput occurences):

Might make sense to create a new component (<FiatEstimate amount="…" /> or so) and use it in the PriceInput component.

Update 2020-02-06

The last screenshot is not up-to-date anymore. For the payment dialog let's implement a UI that allows to select a fiat amount and send the equivalent amount in XLM while still being able to send other assets than XLM.

Move the asset selector down to the end of the dialog, make it a bit more prominent and add a new denomination selector where the asset selector used to be (allowing to set the amount in the token we are about to send or in $, €, …).

Also show the amount of XLM you are about to send (amount field helper text?) if you choose a denomination other than XLM.

The ability to specify the denomination of payments in non-XLM tokens is not mandatory, but might just be a lucky side effect feature.