KCoreDirLister: un-nest Private d-ptr class

Use KCoreDirListerPrivate, and make the d-ptr a unique_ptr, it gets deleted automatically, less book-keeping.

Parent public class pointer is now called "q", more streamlined with modern KDE code (and m_parent makes one think of "parent widget").

Use auto keyword in a couple of places.

Use cbegin()/cend() to prevent detaching. Use iterator.value(), more readable with associative containers.

Get rid of the k prefix for private slots, no Q_PRIVATE_SLOT is used anywhere, so no need for k prefix, IIUC.

KF6 task: https://phabricator.kde.org/T13924