Use non-deprecated KDEInstallDirs variables
Summary:
XDG_APPS_INSTALL_DIR has been marked deprecated.
Use KDE_INSTALL_APPDIR instead.
Reviewers: Kate, cullmann
Reviewed By: Kate, cullmann
Subscribers: cullmann, kwrite-devel
Tags: Kate
Differential Revision: https://phabricator.kde.org/D24860