qt: allow dragging, copying and saving QR code

Add context menu and drag handling to QR code widget.

src/qt/receiverequestdialog.h

@@ -3,12 +3,32 @@
 
 #include <QDialog>
 #include <QImage>
+#include <QLabel>
 
 namespace Ui {
     class ReceiveRequestDialog;
 }
 class OptionsModel;
 
+/* Label widget for QR code. This image can be dragged, dropped, copied and saved
+ * to disk.
+ */
+class QRImageWidget : public QLabel
+{
+    Q_OBJECT
+
+public:
+    explicit QRImageWidget(QWidget *parent = 0);
+    QImage exportImage();
+
+public slots:
+    void saveImage();
+    void copyImage();
+
+protected:
+    virtual void mousePressEvent(QMouseEvent *event);
+};
+
 class ReceiveRequestDialog : public QDialog
 {
     Q_OBJECT
@@ -23,7 +43,6 @@ private slots:
     void on_lnReqAmount_textChanged();
     void on_lnLabel_textChanged();
     void on_lnMessage_textChanged();
-    void on_btnSaveAs_clicked();
 
     void updateDisplayUnit();
 
@@ -31,7 +50,6 @@ private:
     Ui::ReceiveRequestDialog *ui;
     OptionsModel *model;
     QString address;
-    QImage myImage;
 
     void genCode();
     QString getURI();