Add File -> Open Example to ginga

[cdeil]

Would it be possible to add a File -> Open Example option to ginga that contains a handful of nice images (one RADEC, one Galactic, one Allsky, one very wide, one that looks better with log scale)? Then ginga would simply download the images from the web, i.e. they wouldn't be part of ginga itself.

This is very nice to have for new users to try out ginga quickly to see what it can do. E.g. FIJI is doing this and then using the same images for the tutorials (see screenshot below).