[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

This is an attempt to make payment requests much more usable via our GUI and already addresses some of the problems mentioned in #2936. I'm unsure, if #3095 also belongs here?

This is considered a preview and open for feedback and bug reports!

New insecure pr UI:
![insecure pr UI](https://f.cloud.github.com/assets/1419649/1416328/287d82c8-3f4a-11e3-9c7b-c9d984d2e5cb.png)
![insecure pr sendcoins UI](https://f.cloud.github.com/assets/1419649/1399683/2b7f1028-3cba-11e3-93fd-cf2d28676b3c.png)

Todo:
- re-add delete button for payment entries in sendcoinsdialog