[Qt] Rework of payment request UI (mainly for insecure pr)

- this shows insecure (unsecured) payment requests in a new yellowish
  colored UI (based on the secure payment request UI) instead of our
  normal payment UI
- allows us to receive paymentACK messages for insecure payment requests
- allows us to handle expirations for insecure payment request
- changed walletmodel, so that all types of payment requests don't touch
  the addressbook
payment-request UI: use SendCoinsRecipient.message for memo
make processPaymentRequest() use a single SendCoinsRecipient

- as one this pulls main purpose is to change a payment request to
  be displayed as a single sendcoins entry
rework an ugly hack in processPaymentRequest()

- use a QStringList to store valid addresses and format them for GUI and
  debug.log usage via .join()
Merge pull request #3145

395d0d5 rework an ugly hack in processPaymentRequest() (Philip Kaufmann)
952d2cd make processPaymentRequest() use a single SendCoinsRecipient (Philip Kaufmann)
983cef4 payment-request UI: use SendCoinsRecipient.message for memo (Philip Kaufmann)
c6c97e0 [Qt] Rework of payment request UI (mainly for insecure pr) (Philip Kaufmann)
qt: keep a list of requested payments

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?)