Add TabFocus to focusPolicy instead of overriding it

Overriding the previous focus policy may reduce the number ways the widget can receive focus, leading to problems. For example, in Konqueror, it lead to the user being unable to give focus to the location bar by simply clicking on it (see https://mail.kde.org/pipermail/kde-devel/2023-October/002082.html). To avoid this, TabFocus should be added to the existing focus methods.