Add payment request creation to Buy Names tab

[JeremyRand]

To facilitate trading/gifting pre-registrations, we should make the following GUI changes to the Buy Names tab:

1. Below the "Register Name..." button, add a "Request Sale/Gift Registration" button next to an "Extra Amount" textbox.
2. When that button is clicked, call name_new, with an explicit destination address, and unsigned=True, note the output.
3. Then, the same Request/QR-Code UI that normally appears in the Receive tab should appear, but with an extra commitment field in the URL. This field should be populated with the output of name_new's commitment field (the transaction that name_new produces should be thrown away). The amount field in the URL should be 0.01 NMC plus the "Extra Amount".

[robertmin1]

Addressed here, might require improvements: https://github.com/namecoin/electrum-nmc/pull/371