T21b and T21c: Documents, reading and knowledge

- DocumentViewer: pages the app drew on a desk, the page under the view's middle
  counted, zoom steps and fit, thumbnails, notes pinned where a press lands, and
  the owner's FindWidget over the pages, hits lit and the current one revealed
- PageThumbnailList, EpubReader (chapters, type size, progress), PrintPreview
- MarkdownRenderer feeds TableOfContents through sections and outline
- Footnote, TooltipTerm, Glossary, ReadingProgress, ZenMode
- Backlinks, Favorites, TrashBin, PageCover, PageIcon, TemplatePicker,
  VersionHistory with PageHistoryDiff
- editor::FindWidget shows the replace fold and option toggles only with handlers
- block editor tests move to documents/blocks
- icons: a-arrow-down, a-arrow-up
- gallery: reading and knowledge sections; scripted find, changes and zen

src/primitives/icon.rs

@@ -33,6 +33,8 @@ macro_rules! icons {
 }
 
 icons! {
+    AArrowDown => "a-arrow-down",
+    AArrowUp => "a-arrow-up",
     Accessibility => "accessibility",
     Activity => "activity",
     AlarmClock => "alarm-clock",