[ChangeLog][Printing] Printing will now include the
CSS background of the printed elements by default. This restores
the default behavior from Qt WebKit. It can be controlled via the
PrintElementBackgrounds web setting.

Task-number: QTBUG-54918
Change-Id: I37c37456dfec6368f22c36c515ac3cad1bbf8006
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>