[QLiteHtml] Allow access to "litehtml::document::ptr" or provide additional features

I bundle playground/qlitehtml in my project.

https://github.com/martinrotter/rssguard/pull/669

So far it lacks some features I would like to have:

• QLiteHtml has "QLiteHtmlWidget::ResourceHandler" to download embedded resources like pictures or CSS files. It is impossible to tell which case happened as "QLiteHtmlWidget::ResourceHandler" only has "QUrl" as parameter, it should also have parameter to distinguish if this particular request is "picture" or "CSS" - litehtml does provide that information.
• "QLiteHtmlWidget::contextMenuEvent" raises "QLiteHtmlWidget::contextMenuRequested" signal which includes "QUrl" of link (if it was clicked), but I need more information - if image was clicked, etc. -> I want to show more rich context menu with options like "Copy image" etc.
• allow user to specify own "master CSS" or completely remove "master CSS"

I would like to use your upstream QListHtmlWidget, I don't want to completely reimplement it.

I could provide patches to implement the functionality if needed.