Use ecm_setup_qtplugin_macro_names (needs ECM >= 5.45)

==FOR THE TIME WHEN OKULAR HAS MIN ECM VERSION >= 5.45==

Avoids all the explicit boilerplate needed, ensures the dependency on the JSON file ia also set and allows simple export of the setup in the installed CMake config file, so other projects importing the config file have the respective CMake macros set as recommended.

Test Plan: Still builds as before. Touch a plugin metadata JSON file now triggers a moc rerun. CMake config file contains setup of variables.