QWebEngineView doesn't set proper mime types for drag&drop events

If you drag elements from a QWebEngineView to other applications the mime types of the drag event are too restrictive.

Dragging a Link (<a>) will get a mime type of `("text/plain")` instead of `("text/uri-list", "text/html", "text/unicode", "text/plain")`

Therefore it is not possible to drop a link in an applications which only accepts urls (e.g. a media player).

A dragged image also only gets a mime type of `("text/html")`.

I attached a sample application with a QWebEngineView and a drop zone.
It outputs the mime type of drag&drop events.