qt: Disable requests context menu actions when appropriate

The recent requests table will allow you to copy data points even if they do not exist.
This PR implements checks to disable the 'copy label', 'copy message', and 'copy amount' context menu action if the respective fields are empty.

src/qt/receivecoinsdialog.h

@@ -53,6 +53,9 @@ private:
     Ui::ReceiveCoinsDialog *ui;
     WalletModel *model;
     QMenu *contextMenu;
+    QAction* copyLabelAction;
+    QAction* copyMessageAction;
+    QAction* copyAmountAction;
     const PlatformStyle *platformStyle;
 
     QModelIndex selectedRow();