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