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/guiconstants.h

@@ -25,4 +25,10 @@ static const int STATUSBAR_ICONSIZE = 16;
  */
 static const int TOOLTIP_WRAP_THRESHOLD = 80;
 
+/* Maximum allowed URI length */
+static const int MAX_URI_LENGTH = 255;
+
+/* QRCodeDialog -- size of exported QR Code image */
+#define EXPORT_IMAGE_SIZE 256
+
 #endif // GUICONSTANTS_H