Add QR code display on the receive page

This embeds MIT licenced code from Project Nayuki:
https://www.nayuki.io/page/qr-code-generator-library
The C++ part of the code is embedded in src/QR-Code-generator.
See src/QR-Code-generator/Readme.markdown for more info.

src/libwalletqt/QRCodeImageProvider.cpp

@@ -0,0 +1,17 @@
+#include "QrCode.hpp"
+
+#include "QRCodeImageProvider.h"
+
+QImage QRCodeImageProvider::requestImage(const QString &id, QSize *size, const QSize &requestedSize)
+{
+  using namespace qrcodegen;
+
+  QrCode qrcode = QrCode::encodeText(id.toStdString().c_str(), QrCode::Ecc::MEDIUM);
+  QImage img = QImage(qrcode.size, qrcode.size, QImage::Format_Mono);
+  for (int y = 0; y < qrcode.size; ++y)
+    for (int x = 0; x < qrcode.size; ++x)
+      img.setPixel(x, y, qrcode.getModule(x, y));
+  if (size)
+    *size = QSize(qrcode.size, qrcode.size);
+  return img;
+}