Add a QR code generator example

Bundle a beginner Desktop example with Objo Studio that generates QR codes from text or a URL. The example provides a field for the text to encode, a Generate button, a scale choice for the code's pixel density, an ImageViewer that displays the generated code, and an export command that saves the code to disk as a PNG. It demonstrates the QRCode.ToPicture API introduced for generating QR codes without an online service.