[Documentation - QtHelp] Allow navigating with mouse forwards and backwards button v2

This is a fixed and completed version of !44 (closed). It starts with the 3 @davidre's commits rebased on current master. See commit messages in my two commits for details.

Note that currently documentation view zoom GUI is broken because of changes in Qt WebEngine and KTextEditor. I've fixed it in https://invent.kde.org/igorkushnir/kdevelop/-/commits/fix-documentation-zoom, but had to base that branch on this one, because it depends on @davidre's changes here. I'll wait until this merge request makes it to master before creating the fix-documentation-zoom merge request to avoid conflicts.