pull request from rlottie-2team

[arduinho]

feature

1. custom api creator
   you can add lottie customize api without fixing JS, HTML
2. save & upload API list
   custom apis are managed by CppAPI.json file, so you can save created api and load it whenever you want
3. other features:

• reverse-playing
• frame list : you can see the frames and explore lottie animation with clicking
• layer list : lottie file key-path list.
• background controller : you can add image file or change color to background