Function conditionalHeader

docs/app/js/sanddance-app.js:10790–10794

(condition, element, header)

    conditionalHeader(!!legend.children.length, legend, headers.legend);
}
function conditionalHeader(condition, element, header) {
    var existing = existingHeader(element, header);
    if (condition && !existing) addHeader(element, header);
    if (!condition && existing) existing.remove();
}
function addHeader(element, header) {
    const h = document.createElement("h4");
    h.innerHTML = header;