update QRCodeDialog

- remove unused #include <QDebug> and lblBTC label
- update Bitcoin input field to a BitcoinAmountField to allow Bitcoin unit selection
- use BitcoinUnits::format for the resulting amount parameter in the generated URI (always use BTC as per BIP21)
- move MAX_URI_LENGTH and EXPORT_IMAGE_SIZE to guiconstants.h
- add OptionsModel in AddressBookPage and use it in on_showQRCode_clicked() to pass it to QRCodeDialog
- add OptionsModel in QRCodeDialog to enable display unit updates
- add updateDisplayUnit() slot to be able to imediately update currently set bitcoin unit
- make all labels in the UI-file plain text
- resize dialog to match for an updated layout (fields are now stacked and new field)
- remove unused parameters from private slots
- only enable save button, when QR Code was generated
- show message when entered amound is invalid
- add read-only QPlainTextEdit field to output generated URI

src/qt/qrcodedialog.h

@@ -7,6 +7,7 @@
 namespace Ui {
     class QRCodeDialog;
 }
+class OptionsModel;
 
 class QRCodeDialog : public QDialog
 {
@@ -16,22 +17,25 @@ public:
     explicit QRCodeDialog(const QString &addr, const QString &label, bool enableReq, QWidget *parent = 0);
     ~QRCodeDialog();
 
-private slots:
-    void on_lnReqAmount_textChanged(const QString &arg1);
-    void on_lnLabel_textChanged(const QString &arg1);
-    void on_lnMessage_textChanged(const QString &arg1);
-    void on_btnSaveAs_clicked();
+    void setModel(OptionsModel *model);
 
-    void on_chkReqPayment_toggled(bool checked);
+private slots:
+    void on_lnReqAmount_textChanged();
+    void on_lnLabel_textChanged();
+    void on_lnMessage_textChanged();
+    void on_btnSaveAs_clicked();
+    void on_chkReqPayment_toggled(bool fChecked);
+
+    void updateDisplayUnit();
 
 private:
     Ui::QRCodeDialog *ui;
+    OptionsModel *model;
+    QString address;
     QImage myImage;
 
-    QString getURI();
-    QString address;
-
     void genCode();
+    QString getURI();
 };
 
 #endif // QRCODEDIALOG_H