[plot] A single setActiveItem/activeItemChanged anyway the item kind

[vallsv]

Create an API to set an active item, and an event to know the last active item selected.

This could be implemented as an helper or as the underlying architecture.

[t20100]

That would be great, but that's also tricky to make it work along with current status of having an active item for each type... But that would simplify quite a few things I think.