Keep a list of requested payments in the Receive tab so that a user can
recall previously created requests after closing their windows.
Currently this list is not stored between bitcoin-qt sessions. This can
be implemented later, but it is not clear where it should be stored as
I don't think it belongs in the wallet (maybe in QSettings?)
Simplifies the dialog (makes it look less crowded) as well
as the code and makes it possible to copy multiple fields at once.
Also format bitcoin URI as URI, add copy button for URI.
