Fix installation folder for qm files in KDE Apps release settings

Summary:
Old CMake code for estimating the installation dir of the qm files
was broken, did not match where the code is looking and thus the
traditional path as used before by the separate KDE Application
translation release tarballs as well code using libmarblewidget
and explicitely loading the translation with ecm_create_qm_loader,
as it is needed.

Test Plan:
Works now for installation on Linux.
Android only theoretically tested ;)
Windows & macOS... no idea

Reviewers: rahn, nienhueser, ltoscano

Reviewed By: nienhueser

Differential Revision: https://phabricator.kde.org/D5792