Embed CSS code into HTML instead of injecting via JavaScript

This improves performance and eliminates flickering when large man pages
are loaded.

Remove mostly obsolete CSS workarounds in
QtHelpDocumentation::setUserStyleSheet(), which where implemented in
245f8e98 10 years ago. Would be a pity
if these practically useless workarounds caused page flickering.
Rewriting this code in a way that prevents flickering just to improve
the looks of documentation pages from older Qt versions does not seem
worthwhile. Especially since, as far as I can tell, overriding CSS has
never worked with Qt WebEngine, until my recent commit. And all these
years no one bothered to fix it.

If overriding CSS in QtHelp pages is ever needed again, it can be
embedded into a page directly as is done in ManPageDocumentation. In
HelpNetworkReply::HelpNetworkReply(), inside the
`if (request.url().fileName().endsWith(QLatin1String(".html")))` block,
add this code:
    constexpr char headEndTag[] = "</head>";
    const auto headEndTagPos = data.indexOf(headEndTag, 0);
    if (headEndTagPos == -1) {
        qCWarning(QTHELP) << "missing" << headEndTag << "on the HTML page.";
    } else {
        auto cssCode = QByteArrayLiteral("html { background: white !important; }\n");
        if (request.url().scheme() == QLatin1String("qthelp")
            && request.url().host().startsWith(QLatin1String("com.trolltech.qt."))) {
            cssCode += ".content .toc + .title + p { clear:left; }\n"
                       "#qtdocheader .qtref { position: absolute !important; top: 5px !important; right: 0 "
                       "!important; }\n";
        }
        data.insert(headEndTagPos, "<style>" + cssCode + "</style>");
    }

I have verified that this proof-of-concept code works in practice by
replacing "background: white" with "background: green". However, a
proper implementation would have to search for headEndTag in QByteArray
data case-insensitively, which could be done with
std::boyer_moore_horspool_searcher and a custom equality BinaryPredicate
based on <cctype>.

Keep no longer used StandardDocumentationView::setOverrideCss*() because
CSS code cannot be embedded into an external website page. For example,
if kdev-php needs to override CSS in the future, it would have to use
this KDevPlatform API.