Use ecm_setup_qtplugin_macro_names

Summary:
Avoids all the explicit boilerplate needed, consistent with other KF places
and might catch automatically future changes meeded to adapt to other CMake
changes

Reviewers: Frameworks, broulik, davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28636