Add a QWebEngineUrlSchemeHandler which uses KIO::get to produce HTML code to be sent using QWebEngineUrlRequestJob::reply. This scheme handler is added whenever WebEnginePart::openUrl is called for an URL for which KProtocolManager::defaultMimetype returns text/html.
The basic idea is taken from the implementation used in the webengine_stream branch, but instead of changing the way KonqView::openUrl works, it only adds the appropriate scheme handler.
The main issue for this implementation to work is that QWebEngine refuses to load local resources in the generated HTML (images and CSS files) because of cross-origin rules. The only way to work around this limitation that I could think of is to parse the HTML code produced by KIO::get and to replace, in img and link elements the URL with a data URL embedding the content of the file. To do this, the scheme handler uses the external program htmltidy to convert the HTML generated by KIO into XHTML, then uses QDomDocument to parse the resulting XHTML file and replace the URLs appropriately. If one of these two steps fails, the original HTML will be used: this means that the user won't be able to see most of the formatting in the man/info pages but will be able to read the text.
Unfortunately, there's still a problem I couldn't solve: some resources in the HTML produced by KIO use a help URL rather than the standard file one, but I couldn't find out to which file they point, so they aren't changed, meaning that the page is not displayed completely as it was intended.