Folder facade

addFolder(options?) creates a jolly-folder. A folder is a container, with the same add*() methods as the pane.

interface FolderOptions {
  title?: string; // the folder label
  expanded?: boolean; // the folder open state, default true
}

const state = { visible: true };
const folder = pane.addFolder({ title: "Layer" });
folder.addBinding(state, "visible");

state.visible = false;
folder.refresh();