[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