The old traders could lookup both applications and plugins, but plugins
should nowadays be loaded via their JSON file, no desktop-file trader
To get rid of plugins and servicetypes from the API, and keep
only the application-lookup-related code, the older traders will be
deprecated and their users should either switch to JSON for plugins (KPluginLoader)
or KApplicationTrader for application desktop files.
One API question remains:
KApplicationTrader::self()->query("...") like the old traders, or
KApplicationTrader::query("...") (static methods only, or making it a namespace, a bit procedural rather than OO), or
KApplicationTrader().query("...") like QMimeDatabase/QFontDatabase (but with default ctors for performance) ?