Use KPluginMetaData to load KCMs in KCMultiDialog

Task: https://phabricator.kde.org/T14501
2 jobs for master in 8 minutes and 30 seconds (queued for 89 minutes and 6 seconds)
Status Name Job ID Coverage
  Build
passed build_clazy_clang_tidy #112212

00:05:22

passed linux kf5-qt5 qt5.15 #112211

00:03:07

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