Use kcoreaddons_add_plugin

This patch steals idea from D9710.
The difference in regard to the other patch is:

1. right .json file is used for every plugin,
2. no redundant paths are added in code (only build system is modified),
3. no two different paths for installing plugins,
4. all plugins get stored in bin directory after build.

More about it can be red on
https://github.com/KDE/kcoreaddons/blob/master/KF5CoreAddonsMacros.cmake

Loading plugins without plugins path set is not working for me, but I
think it should work for someone according to
http://doc.qt.io/qt-5/qcoreapplication.html#libraryPaths

Differential Revision: https://phabricator.kde.org/D11160