Doc: Add inheritance to WebEngineView type docs

...to display all properties available for the type.

Task-number: QTBUG-59673
Change-Id: I0ee61abefcfbcc229812dc7c3875d16fb279f9e0
Reviewed-by: Viktor Engelmann <viktor.engelmann@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Michael Brüning <michael.bruning@qt.io>