Port settings to KPluginMetaData

- Use KPluginLoader to load the KCMs
- Clean up unneeded parts of KCMUtils classes, like rootOnlyMMessage
- Sort plugins beforehand, this saves us the trouble of inserting them
at their correct position
- Drop code paths for KPluginInfo and KCModuleInfo
- Drop support for setdlg files, instead use the KPLuginMetaData from
the Kontact plugin to store the information.
- Port away from KPluginInfo::kcmServices, see https://phabricator.kde.org/T13555#259170