＊ (first / parent, levels: 1) ├─＊ (node / parent, levels: 2) │ └─＊ (last in its own branch, levels: 3) └─＊  (last, levels: 2)
function getTreeLine(treeNode, addClass) {
        var cls = 'item-tree-line';
        if (addClass) cls += ' ' + addClass;
        if (treeNode === 'parent') cls += ' item-tree-parent';
        return '<img class="' + cls + '" src="img/tree-' + treeNode + '.png" width="' + app.TREE_NODE_WIDTH + '" height="' + app.SIDEBAR_NODE_HEIGHT + '" />';
    }