Page display pointer: Considerably improved accessibility

- Updated pointer to move within content DOM so that you can back-focus
  into the pointer if desired.
- Added new "Section select mode" which toggles focusabiltiy for main
  content sections, with ability to show pointer via enter press on
  these.
- Updated pointer with proper input/button labelling.

Tested via orca screen reader on Firefox/Fedora/Gnome.
For #3975

resources/js/components/page-display.js

@@ -3,7 +3,7 @@ import {scrollAndHighlightElement} from '../services/util';
 import {Component} from './component';
 
 function toggleAnchorHighlighting(elementId, shouldHighlight) {
-    DOM.forEach(`a[href="#${elementId}"]`, anchor => {
+    DOM.forEach(`#page-navigation a[href="#${elementId}"]`, anchor => {
         anchor.closest('li').classList.toggle('current-heading', shouldHighlight);
     });
 }