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?)

src/qt/walletmodel.h

@@ -18,6 +18,7 @@
 class AddressTableModel;
 class OptionsModel;
 class TransactionTableModel;
+class RecentRequestsTableModel;
 class WalletModelTransaction;
 
 class CCoinControl;
@@ -88,6 +89,7 @@ public:
     OptionsModel *getOptionsModel();
     AddressTableModel *getAddressTableModel();
     TransactionTableModel *getTransactionTableModel();
+    RecentRequestsTableModel *getRecentRequestsTableModel();
 
     qint64 getBalance(const CCoinControl *coinControl = NULL) const;
     qint64 getUnconfirmedBalance() const;
@@ -160,6 +162,7 @@ private:
 
     AddressTableModel *addressTableModel;
     TransactionTableModel *transactionTableModel;
+    RecentRequestsTableModel *recentRequestsTableModel;
 
     // Cache some values to be able to detect changes
     qint64 cachedBalance;