feat: add QRCodeWidget

[sassanh]

This pull request adds a new widget QRCodeWidget which can be used to display QR codes in the app.

[mehrdadfeller]

Looks good to me. Please go ahead and merge it. Since GUI knows the screen resolution, in the future it might make sense to add a check to see if the provided string can be encoded into a QR code given the screen resolution and return an error if the input string is too long.

[sassanh]

@mehrdadfeller Please note that this is just a widget, it can be rendered with any arbitrary size (smaller than the screen size) it would be like expecting text labels to check if they can fit in the screen with their current text and font size.
But the concern is indeed an important one, we can add minimum width and minimum height to this widget in the future, or just like Kivy's Label widget, we can add texture_size property so that the parent widget has a way to be notified when it doesn't fit.