[#279] added new unknown app icon

<div style="
    width: 50px;
    height: 50px;
    background: #888;
    margin: 50px auto;
    font-size: 36px;
    line-height: 50px;
    text-align: center;
    text-shadow: rgba(0,0,0,0.5) 1px 1px 1px;
    color: #ccc;
">?</div>