Use ecm_setup_qtplugin_macro_names

Summary:
Avoids all the explicit boilerplate needed and allows
code reusing.

Depends on D10749

Test Plan:
Still builds, generated and installed CMake Config file works
as before.

Reviewers: Plasma, Frameworks

Subscribers: apol

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11295