Derive plugin id from filename, clean up json files

We ignore the name/description and only display the id in the ui.
To make sure the is are the same, the OUTPUT_NAME cmake target property is set.
Otherwise, we would need a json file with the KPlugin object and then the id in it, which is
unnecessary work for sth. that can be done in a single line.

Having the file base name and id differ is also discouraged, see https://phabricator.kde.org/T14499

framebuffers/pipewire/CMakeLists.txt

@@ -58,6 +58,7 @@ if (HAVE_DMA_BUF)
     )
 endif()
 
+set_target_properties(krfb_framebuffer_pw PROPERTIES OUTPUT_NAME pw)
 install (TARGETS krfb_framebuffer_pw
          DESTINATION ${KDE_INSTALL_PLUGINDIR}/krfb/framebuffer
 )