Avoid flicker when highlighting a section

Avoid flicker when highlighting a section

Previously, elements were wrapped in a div on hover and unwrapped when
the mouse was moved again. This generally worked okay, but led to
problems where the wrapped section contained elements with their own
rendering context (video, object, iframe, ...). The wrapping caused
rerendering of these elements resulting in flicker and layoutshift:

splitbrain/dokuwiki-plugin-vshare#13
cosmocode/dokuwiki-plugin-diagrams#4

This patch changes the higlighting mechanism to always wrap *all*
sections in a div and only toggle classes on hover. Class names have
been chosen to be backwardscompatible, so this should continue to work
as before in all templates.