Display links to anchors in view mode on hover

In view mode, add the ability to hover over a heading with the mouse and display an anchor icon, in order to get a link with an anchor or copy the anchor id in the clipboard (so that it can be pasted when editing a page and linking to some heading). Something similar to:

• https://vuejs.org/guide/essentials/application.html
• https://ge.xwiki.org/s/fooar7yu3hqs4

See https://forum.xwiki.org/t/getting-heading-links-and-anchor-ids-more-easily/14185