planworkpart: install into kf5/parts subdir
- 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