[4.4.3] Added image and actions to musesound update spec

[igorkorsukov]

New spec

    /*
    {
        "version": String,
        "image_url": String,  // it can be base64 data, like "data:image/png;base64,iVBORw0KGgoA......"
        "content": {
            "locale_code": {
                "notes": String,
                "features": [String]
                "action_title": String // title of action button
            }
        },

        // open app or web page url, try in order,
        // like this ["musehub://?from=musescore", "muse-hub://?from=musescore", "https://www.musehub.com"]
        "actions": {
            "windows": [String],
            "macos": [String],
            "linux": [String]
        }
    }
    */

[RomanPudashkin]

I'm not sure we need it in this interface... Perhaps it was originally intended to be reusable, but with the addition of the actions argument, it's unclear how to use it. We can make it a private method of MuseSoundsCheckUpdateScenario

[RomanPudashkin]

Maybe just remove it?