Coordinates in cursor display may not be RA/Dec

I noticed while working on astropy/astrowidgets#142 that although the cursor is always labeled as RA/Dec it is not necessary actually RA/Dec. In the ginga implementation the sample image has a WCS in *galactic* coordinates and that is what the cursor displays. I don't know that that is wrong, necessarily, but it is incorrect to label it as RA/Dec.

Seems like there are a couple of options:

1. always display RA/DEC (ICRS? other?), converting as necessary.
2. Display whatever coordinates the WCS is in, with appropriate names for the lat/lon.

@pllim -- you may want to check what the `imviz` viewer currently does. This question above is probably an API question rather than an implementation decision.

Start by reviewing the cursor coordinate labeling in the API and compare the ginga implementation with the imviz viewer, as suggested in the issue. Decide whether the API should convert to a standard RA/Dec system or expose the WCS coordinate names, then define the expected cursor labels for galactic and other coordinate systems.

Written by the indexing model from the issue text.