Fonts KCM: Fix layout of hinting settings, port away from KDE4

The dialogue spacing around and between GUI elements is restored,
and the combo boxes lined up.

KDE4 -> KF5 porting done:

  KDialog -> QDialog
  KApplication -> QApplication
  KProcess -> QProcess
  KStandardDirs -> KSharedConfig
  KDoubleNumInput -> QDoubleSpinBox

KDE4 porting still remaining to be done:

  KGlobalSettings::emitChange(KGlobalSettings::FontChanged)
  KFontDialog::getFontDiff()

Differential Revision: https://phabricator.kde.org/D4740