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/addresstablemodel.cpp

@@ -297,7 +297,7 @@ QVariant AddressTableModel::headerData(int section, Qt::Orientation orientation,
 {
     if(orientation == Qt::Horizontal)
     {
-        if(role == Qt::DisplayRole)
+        if(role == Qt::DisplayRole && section < columns.size())
         {
             return columns[section];
         }