qt: keep a list of requested payments

Continuation of work in #3099.

Keep a list of requested payments in the Receive tab so that a user can recall previously created receive 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 here it should be stored as I don't think it belongs in the wallet (maybe in QSettings?)

![recently_requested](https://f.cloud.github.com/assets/126646/1475793/dc78e68a-4640-11e3-812b-3f39978099c4.png)