Update demo app

This is only to improve the UI of our demo app.

What's changed

• added a price variable PXLProduct
• improved the design of grid and linear list
• Improved the video player and the image viewer and added a product list at the bottom
• added an animated Pixlee loading icon. (the look is made by a JSON file) I used Lottie library to do this
  • Lottie: (https://lottiefiles.com/)
    • available: Android, iOS, Web, React Native, and Windows