Support also translated doc added in the KDE Application tarball

Summary:
This is a follow-up of D5648, where the translatations (both native
and KI18n-based for the Frameworks applications) are now handle
directly in the tarball. This change add the last bit (support for
translated user documentation).

Test Plan: Only tested compilation without po/

Reviewers: kossebau, rahn, nienhueser

Reviewed By: nienhueser

Differential Revision: https://phabricator.kde.org/D5738