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.cpp

@@ -7,6 +7,7 @@
 #include "addresstablemodel.h"
 #include "guiconstants.h"
 #include "transactiontablemodel.h"
+#include "recentrequeststablemodel.h"
 
 #include "base58.h"
 #include "db.h"
@@ -26,6 +27,7 @@
 WalletModel::WalletModel(CWallet *wallet, OptionsModel *optionsModel, QObject *parent) :
     QObject(parent), wallet(wallet), optionsModel(optionsModel), addressTableModel(0),
     transactionTableModel(0),
+    recentRequestsTableModel(0),
     cachedBalance(0), cachedUnconfirmedBalance(0), cachedImmatureBalance(0),
     cachedNumTransactions(0),
     cachedEncryptionStatus(Unencrypted),
@@ -33,6 +35,7 @@ WalletModel::WalletModel(CWallet *wallet, OptionsModel *optionsModel, QObject *p
 {
     addressTableModel = new AddressTableModel(wallet, this);
     transactionTableModel = new TransactionTableModel(wallet, this);
+    recentRequestsTableModel = new RecentRequestsTableModel(wallet, this);
 
     // This timer will be fired repeatedly to update the balance
     pollTimer = new QTimer(this);
@@ -325,6 +328,11 @@ TransactionTableModel *WalletModel::getTransactionTableModel()
     return transactionTableModel;
 }
 
+RecentRequestsTableModel *WalletModel::getRecentRequestsTableModel()
+{
+    return recentRequestsTableModel;
+}
+
 WalletModel::EncryptionStatus WalletModel::getEncryptionStatus() const
 {
     if(!wallet->IsCrypted())