Fix primary view changing folder for the closing animation

This commit fixes a bug which made it so an UrlNavigator that was
disconnected from a viewContainer would still indirectly control
the viewContainer because of a dubious connection created in
DolphinMainWindow.

Instead the internal UrlNavigator of the viewContainer is used
which is always connected to its own viewContainer.

This way the the supposedly disconnected primary UrlNavigator will
actually not affect the old disconnected viewContainer that is
currently being closed.

BUG: 437080
FIXED-IN: 21.08