Port away from deprecated kcoreaddons_desktop_to_json
This basically just does what the warning messages emitted during configuring stage tell:
- For collections, storage, importers, services and scripts, convert .desktop files to .json in source using desktoptojson (leaving the .desktop files in place, too)
- For context applets, convert metadata.desktop to .json and remove .desktop
The now unneeded kcoreaddons_desktop_to_json lines are removed from the CMakeLists.txt files
(cherry picked from commit 914dd973)