QR code fixes

New classes ScanQRTextEdit and ShowQRTextEdit.
Reason: dependencies on zbar availability and issues with the QRTextEdit constructor.
- ScanQRTextEdit needs access to the config (fetch camera). It needs to load
the zbar processor properly before trying to scan. Keeping a reference to
the processor in qrscaner fixes the crashes on windows.
- ShowQRTextEdit should not have access to scan_qr().
- no need to setReadOnly anymore. It is clear from the class name.

Show master pub keys now has a Combobox if multiple accounts are
available.

gui/qt/qrcodewidget.py

@@ -114,7 +114,7 @@ class QRDialog(QDialog):
 
             def copy_to_clipboard():
                 bmp.save_qrcode(qrw.qr, filename)
-                self.parent().app.clipboard().setImage(QImage(filename))
+                QApplication.clipboard().setImage(QImage(filename))
                 QMessageBox.information(None, _('Message'), _("QR code saved to clipboard"), _('OK'))
 
             b = QPushButton(_("Copy"))