Implement layers (z-index) for datasets

[kurkle]

Resolves: #6050
Resolves: #5171

Pen

[kurkle]

Not quite sure where to document this feature.

[benmccann]

The mixed charts docs are probably the only docs that talk about putting multiple datasets in a chart, so that might be the place to talk about the relative orderings?

Will the docs from #6241 also need to be updated? I.e. where it says Values < 0 are drawn under datasets, > 0 on top.

[kurkle]

Will the docs from #6241 also need to be updated? I.e. where it says Values < 0 are drawn under datasets, > 0 on top.

No, that still applies. This allows changing z-order between datasets (and allows multiple layers for dataset, but none such are implemented right now, line could draw points on a separate layer for example)

Filler plugin does not follow this z-order (it was changed to draw before all datasets).
It would need the new order, so maybe a getSortedVisibleDatasets or getSortedVisibleDatasetMetas should be made (as public).

[kurkle]

After a short chat with @simonbrunel, going dataset.order route instead. Will PR that simpler solution another day.

[JorisDebonnet]

Just linking to the new issue here: #6268

(but unfortunately that one does not actually take care of the layer / z-index feature, but does something else)