Add the ability to re-order layer zorder via UI

[astrofrog]

At the moment I don't think we have a way to reorder the visible order (zorder) of layers via the UI, so we should make sure that we provide a way to do this, and that all viewers respect the zorder.

[Carifio24]

I did this for the scatter and histogram viewers in #334, but never got around to doing the profile or image viewers.

[astrofrog]

Ah did your PR include a way of reordering the layers in the UI?

[Carifio24]

It did, yeah - there's an IntText in the layer options widget that allows setting the z-order for the currently selected layer. The order of the layers in the dropdown menu will update to reflect z-ordering as well (though currently the lowest z-order layer is at the top of the dropdown, maybe that should be reversed).

It is definitely not as nice of an experience as the drag-and-drop in the Qt app though, having something like that would probably be ideal.