Add focusOnNavigationEnabled setting

The focusOnNavigationEnabled setting allows changing the behavior of
whether a WebEngine view (widget or quick) will automatically get
focus, whenever a navigation action happens (load, reload, previous
history entry, etc).
The default behavior before this patch was to always grab the focus.

[ChangeLog][QtWebEngine][General] Add focusOnNavigationEnabled setting
which allows controlling whether a web view will receive focus on a
navigation request. Previously the view always received the focus.

Task-number: QTBUG-52999
Change-Id: I6d30d973a41b53011131f21dcecbf6ec4d652759
Reviewed-by: Kai Koehne <kai.koehne@qt.io>