Move and delete chapters by hand in the chapter list

The merge cannot always tell where new chapters go. For example, a site
can list an announcement before the chapters of an old book. You can now
select chapters and move them, in their order, to after another chapter
or to the start. You can also delete them with their revisions. A story
keeps at least one chapter. The book is then rebuilt. A deleted chapter
that the site still lists comes back with the next download.

The Move… and Delete… buttons open a dialog that names the selected
chapters before anything changes. A shift-click selects a range of
chapters.

The Select all checkbox moves into the first heading of the chapter
list. On a phone, that heading shows with the text "Select all".

tests/js/flow-helpers.mjs

@@ -30,6 +30,8 @@ export class Element {
   removeAttribute(name) { delete this.attributes[name]; }
   addEventListener(name, fn) { this.listeners[name] = fn; }
   focus() { document.activeElement = this; }
+  showModal() { this.open = true; }
+  close() { this.open = false; }
   remove() { if (this.parent) this.parent.children = this.parent.children.filter((child) => child !== this); }
   get firstElementChild() { return this.children.find((child) => child instanceof Element); }
   get textContent() { return this.children.map((child) => child instanceof Element ? child.textContent : String(child)).join(''); }