API methods to support removing/reordering data layers within a panel

[Frencil]

While the API methods and UI elements for programatically adding, removing, and rearranging panels on a populated LocusZoom plot are fairly mature at this point, analogous methods for data layers are not.

Presently it is possible to programatically add a data layer with Panel.addDataLayer(). That's about it.

What's needed:

• Panel.removeDataLayer()
• Method(s) to alter the relative z indexes of data layers in a panel and "apply" it (i.e. reorder data layer elements in the SVG to achieve the desired visual result)
• UI elements analogous to the panel controls elements to allow for changing the z-index order of data layers in a panel interactively

[Frencil]

This has been implemented as of v0.5.2.