VLM core API to expose VLM schedules

There are queries such as VLM_GET_MEDIAS to get VLM media objects, but those don't include VLM schedules, for which there is no equivalent: there is only a VLM_CLEAR_SCHEDULES accompanied by the comment /* TODO: missing schedule control */ The show command doesn't properly display the useful details of a VLM schedule either. Currently the only option would be to export and then re-parse the whole configuration, which is impractical.

Forked from #17839

changed the description

This is rather unfortunate as the Qt VLM dialog attempts to support VLM schedules, see #25126

This looks very backward to me. We should rather drop or replace schedules. Designing an API around something that badly broken seems like putting the cart before the horse.

Actually I was wrong, calling specifically show <name> returns the schedule properties as a VLM message tree. So there is some usable API, but still not a proper one.

mentioned in issue #25126

added Component::Core: VLM label