[webtoons] play_image and stop_image in background music (#8733)

[ollm]

I've added the keywords play_image, play_image_filename, stop_image, and stop_image_filename to be able to associate background music with the corresponding images. I've only updated the extractor, I haven't modified the tests for now.

Related: #8733

Usage:

{
        "webtoons":
        {
            "filename": {
                "type == 'bgm'": "bgm_{play_image:>04}-{stop_image:>04}.{extension}",
                "": "{num:>04}.{extension}"
            },
            "directory": ["{comic_name}", "{episode_no}"]
        },
}

[mikf]

I've renamed the metadata fields to num_play|_stop & filename_play|_stop. Hope that's OK.

        "webtoons":
        {
            "filename": {
                "type == 'bgm'": "bgm_{num_play:>04}-{num_stop|'':>04}.{extension}",
                "": "{num:>04}.{extension}"
            },
            "directory": ["{comic_name}", "{episode_no}"]
        },

[ollm]

I've renamed the metadata fields to num_play|_stop & filename_play|_stop. Hope that's OK.

It's okay for me 👍