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/receiverequestdialog.cpp

@@ -85,12 +85,10 @@ ReceiveRequestDialog::ReceiveRequestDialog(QWidget *parent) :
 
 #ifndef USE_QRCODE
     ui->btnSaveAs->setVisible(false);
-    ui->btnCopyImage->setVisible(false);
     ui->lblQRCode->setVisible(false);
 #endif
 
     connect(ui->btnSaveAs, SIGNAL(clicked()), ui->lblQRCode, SLOT(saveImage()));
-    connect(ui->btnCopyImage, SIGNAL(clicked()), ui->lblQRCode, SLOT(copyImage()));
 }
 
 ReceiveRequestDialog::~ReceiveRequestDialog()