Move the QPrinter and QtWidgets related code out of the PDFium wrapper

This moves the actual printing using QPrinter to the WebEngineWidgets
part of the API.
The printsupport module depends on the widgets module and therefore
QtWebEngineCore also had a dependency to widgets. This is removed by
this change.

Change-Id: If6e5745709a59de18f2123b930cbe6e64390c867
Reviewed-by: Michal Klocek <michal.klocek@qt.io>