To be consistent with all other media plugins, the parameter name should
be `step`, not `delta`.
A back-compatibility layer has however been introduced for plugins
(namely `music.mpd` and `music.spotify`) that already exposed the old
interface.
- Using `@action` annotation to indicate methods that are allowed to be
executed as actions
- The output and errors of an action are automatically wrapped into a
`Response` object without any response build required on the plugin side