qt: Enable and disable the Show and Remove buttons for requested payments history

based on whether any entry is selected.

src/qt/receivecoinsdialog.h

@@ -10,6 +10,7 @@
 #include <QMenu>
 #include <QPoint>
 #include <QVariant>
+#include <QItemSelection>
 
 namespace Ui {
     class ReceiveCoinsDialog;
@@ -51,6 +52,7 @@ private slots:
     void on_showRequestButton_clicked();
     void on_removeRequestButton_clicked();
     void on_recentRequestsView_doubleClicked(const QModelIndex &index);
+    void on_recentRequestsView_selectionChanged(const QItemSelection &, const QItemSelection &);
     void updateDisplayUnit();
     void showMenu(const QPoint &);
     void copyLabel();