Use imported targets instead of qt5_use_modules

From Qt's documentation: "This macro is obsolete. Use
target_link_libraries with IMPORTED targets instead." It's only
recommended with cmake >=2.8.9 & < 2.8.12. Kube already requires
cmake 3.0.
One advantage of using the imported targets is, that
cmake complains if a target isn't found before it's used, like
Qt5Concurrent missing from the find_package_call here.

Reviewers: Kube, cmollekopf

Reviewed By: Kube, cmollekopf

Subscribers: Kube

Differential Revision: https://phabricator.kde.org/D6362