[Feature] Separate z-level for markarea and markarea label

[hughess]

What problem does this feature solve?

I am creating a markarea which I would like to be behind the gridlines on my chart. I would also like to add a label to the markarea, but I would like that label to be in front of the gridlines.

I can successfully change the z-level of the markarea to get it behind the gridlines (by including it in its own series with type:line), but I cannot control the z-level of the markarea label.

Ideally, you could set the zLevel on both the markArea and the markArea.label elements.

What does the proposed API look like?

series: {
   type: 'line'
   markArea: {
      data: myData,
      label: {
         show: true,
	 zLevel: 3
      },
      zLevel: 0
    }
}