kmplot_part: add JSON metadata to plugin, install into kf5/parts subdir

David Faurerequested to merge
work/dfaure/install_in_kf5_parts into master

Test plan:

  • removed lib64/plugins/kmplotpart.so
  • kmplot still works (finds the part on startup)
  • konqueror still allows to preview a fkt file
  • lxr shows no other code loading "kmplotpart" by name directly

KF6 task: https://phabricator.kde.org/T12340

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