Modernise code base in a couple of classes

Ahmad Samirrequested to merge
work/ahmad/kdiskfreespace-private into master
  • Un-nest Private class and make d-ptr a unique_ptr, KF6 task: https://phabricator.kde.org/T13924
  • Remove Q_PRIVATE_SLOT() usage where possible and remove the k prefix from slot names
  • Prefix member variable names with m_

NO_CHANGELOG

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