[4.0.0] Favorite Currencies

The Problem:

When you attempt to create a new account with a different currency or add a transaction with a non-default currency, the exchange rate synchronization process often takes around 1-2 minutes to complete if the internet connection is poor, and the process would fail, resulting in a "0" outcome.

What I suggest:

Create a "Favorite Currencies" list underneath "Default Currency; allow users to pin their favorite currencies and sync exchange rates at the same time as such rates are generally stable and there would be no need to update them every time a new transaction or account is created with a different currency.

Thank you!