Webcam scanning

This PR adds ability to read pairing payload from QR code via webcam. On Mac OS, the user should be asked to grant system privileges for app to use webcam. This should not apply to other platforms but is needed to be tested properly. If denied, the modal won't open and error message should appear.

If the QR code is read, the modal window closes automatically and applies payload to text input.

See screenshots below:

Closes #19 (closed)