Qt: "Buy Names" tab should detect if name_new already exists

[JeremyRand]

The Buy Names logic for checking if a name already exists doesn't check for existing name_new transactions. It would be useful to check for such transactions, and call name_firstupdate instead of name_autoregister in such cases.

Among other things, this would have practical benefits if the name_new transaction was produced by a 3rd party such as a Bisq counterparty.

[robertmin1]

Addressed here: https://github.com/namecoin/electrum-nmc/pull/371 But we should maybe look at a better way to do this