KUrlNavigator: modernise code base

• Un-nest Private class and make d a unique_ptr, KF6 task: https://phabricator.kde.org/T13924
• declare and initialize member variables at the same time
• Use a delegating ctor, and do away with the now redundant initialize() method
• Remove Q_PRIVATE_SLOT() usage, not needed any more
• Add a constructor for LocationData (tip from dfaure)
• Use QList::constFirst/Last()/at() to prevent unnecessary detaching

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