Add an option to use an UrlNavigator in the toolbar instead

This commit adds a locationInToolbar KToggleAction to switch between
using a location bar to navigate or using a new custom QWidgetAction
in the toolbar.

A big portion of this MR is refactoring because until now the
UrlNavigator was tightly intertwined with the DolphinViewContainer.
With this MR an UrlNavigator for controlling a View can be freely
connected or disconnected with a single method call.

A DolphinUrlNavigator class is created in the process which contains all
Dolphin-specific UrlNavigator code which did previously reside in the
DolphinViewContainer class. Other application parts that belong to
UrlNavigator-management are also moved here.