Youtubecenter: [Feature Request] Add to playlist from subscription page

Very similar to #2107

Video thumbnails should definitely have an add to playlist button (not just on subscription page).

Frustratingly YouTube used to have this, until in 2012 they replaced the add-to-playlist button with the watch-later button in the bottom right of video thumbnails.

So it looks like we only need to make a single ajax post request, which will return the HTML of the Add To menu (well, it returns XML, but the root->html_content element contains the HTML of the menu).

The request is to https://www.youtube.com/addto_ajax?action_get_dropdown=1&hide_watch_later=false, and the body should be x-www-form-urlencoded with video_ids, session_token (and possibly src_playlist_id).

Ok, looks like session_token and src_playlist aren't needed.

The following request worked fine for me:

function getAddMenu(video, domElement) {
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function readyStateChange() {
        if (this.readyState == 4 && this.status == 200) {
            var html = xhttp.responseXML.getElementsByTagName('html_content')[0].textContent;

            domElement.innerHTML = html;
        }
    };
    xhttp.open('POST', '/addto_ajax?action_get_dropdown=1&hide_watch_later=false', true);
    xhttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
    xhttp.send('video_ids=' + video);
}

getAddMenu('rh7kpkwXnwA', document.getElementById('foo'))