[KMenuEdit] port away from KStandardDirs
Test Plan:
only did smoke testing (kmenuedit starts and displays the tree, clicking
on one item shows details).
Reviewers: mlaurent
Reviewed By: mlaurent
Subscribers: plasma-devel
Tags: Plasma
Maniphest Tasks: T11566
Differential Revision: https://phabricator.kde.org/D23823