[Feature Request] Force 8 decimal places on amount and fee values

[ottosch]

In some places, decimal places are trimmed if the last digits are zero. I think this can lead the user to send the wrong amount and is inconsistent with some other places.

For example, the "balance" and "value" columns of "Transactions/UTXOs" grid always display 8 decimal places, but the "send" button and "amount" and "fee" inputs of the send view don't.

I noticed this can be adjusted with setMinimumFractionDigits(8) on UnitFormat::getBtcFormat() for the send button, although "Mempool" label will display "0.00000000 BTC" instead of "0 BTC" (which I'm ok with, but might be undesirable) and on CoinFilter::coinFormat for the text fields.