Fixes the QR code widget to include a 10px margin to offset the widget on dark theme background

On my screen, QR codes are not scannable because I use a dark green theme and there is not an adequate amount of white padding between the background and the QR code.

[QR code without margin (unscannable)](http://i.imgur.com/C1kKXx1.png)

[QR code with margin added (scannable)](http://i.imgur.com/HD5rTKn.png)

It looks like the upstream pyqrnative has a margin parameter, but the one included in electrum does not, so I simply drew a white square underneath the QR code that is 10 pixels bigger.