Function debounce

docs/app/js/sanddance-app.js:106793–106799

(delay, handler)

    return isFunction(_) ? _ : ()=>_;
}
function debounce(delay, handler) {
    let tid;
    return (e)=>{
        if (tid) clearTimeout(tid);
        tid = setTimeout(()=>(handler(e), tid = null), delay);
    };
}
function extend(_) {
    for(let x, k, i = 1, len = arguments.length; i < len; ++i){
        x = arguments[i];