The widget wrapper added for hiding the implementation choice
between QWebEngine or QWebKit broke the custom context menu setting,
as any calls to set contextMenuPolicy of a StandardDocumentationView
object, like in QtHelpDocumentation::documentationWidget(), no longer
affected the web view itself.

This patch restores that (and ensures the basic custom menu also with
non-QtHelpDocumentation users of StandardDocumentationView, like by
plucings for man-pages or cmake).

The change from exposing copyAction() to createStandardContextMenu()
should allow extending the base menu (e.g. by find-text action)
without menu extending classes like QtHelpDocumentation having
to know about it.