Basic UI controls in interactive-figure viewer

[pkgw]

Our new interactive-figure viewer should provide some basic controls so that people can customize their view of the data. In particular, there should be:

• [ ] Ability to change background imagery
• [ ] Ability to change the opacity of the "main" image (keeping in mind that there may be between 0 and N additional images loaded in the serialized view!)
• [ ] Ability to change data scaling parameters (vmin, vmax, stretch) of the "main" FITS image, if any. The UI for this might be a bit tricky since it might not be obvious what range the vmin and vmax values should be allowed to cover
• [ ] Ability to change the colormap used for the "main" FITS image
• [ ] Ability to save FITS file backing the "main" image to disk

The interactive-figure viewer will often be presented in a somewhat cramped format (if used in e.g. AAS journal online articles) so the UI will probably have to be compact; the strategy taken by the aas-timeseries tool is a likely good one to emulate.