VSE: Ability to use Image data block IDs

Posting it here because this is highly requested.

For videos, VSE today can either refer to files on disk (via file path), or to the Movie data block IDs. However for images, VSE can only refer to files on disk via path; ability to use image IDs is missing. People in various channels (devtalk, chat etc.) have expressed need for this. Example use cases: 1) using image that has been edited/painted directly in blender, 2) using a packed image, e.g. a render result that has been packed into the blendfile, 3) using an image produced by texture nodes.

More info here: projects.blender.org/blender/blender/issues/150507

✌