planworkpart: install into kf5/parts subdir

David Faurerequested to merge
work/dfaure/modernize_kparts_usage into master
  • fix name of the library in the part's desktop file
  • install the desktop file for the part Otherwise what's the point in creating a KPart?

Test plan:

  • removed lib64/plugins/calligraplanworkpart.so
  • konqueror allows to right-click on a .planwork file and preview it

But maybe this doesn't need to be a KPart at all? At least this way I can mark another part as ported.

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

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