Assets improvements

[tristan-mouchet]

There are a lot of things related to assets that should be re-implemented on Ycode 2.0:

• Asset layers (Image, Icon, Audio, Video, Youtube),
• Asset settings for each type of asset layer,
• Asset manager UI to be connected and to use real data (folders, previews, selection),
• Link to assets (links have not been implemented yet),
• Asset data fields in collections (document, image, multi-image, audio, video, youtube),