This commit adds a showLocationBar action to toggle between
using a location bar to navigate or using a new custom QWidgetAction.
The KUrlNavigator of this new QWidgetAction synchonises itself with
the KUrlNavigator of the active view whenever changes occur to either
of them.
When a separate location bar is already present and this QWidgetAction
is on a toolbar it will act like an empty expanding spacer but will
internally mimic the state of the KUrlNavigator of the active view.
When no separate location bar is present setWidgetVisible() is used
to show the internal KUrlNavigator. This switch is done by a
QStackedWidget which is the defaultWidget() of the QWidgetAction.