Enhancements to the Catalogs plugin

	modified:   ginga/Catalog.py
	modified:   ginga/FitsImage.py
	modified:   ginga/VOCatalog.py
	modified:   ginga/gtkw/ColorBar.py
	modified:   ginga/gtkw/GtkHelp.py
	modified:   ginga/gtkw/plugins/Catalogs.py
	modified:   ginga/misc/plugins/CatalogsBase.py
	modified:   ginga/qtw/ColorBar.py
	modified:   ginga/qtw/plugins/Catalogs.py
	modified:   ginga/wcs.py

- Added controls to change the color map used for plotting rings around sources
- Catalog searches can now return different columns and these will be shown
    after the first few "standard" columns
- Added a control so user can select which column is used to color code
    plotting rings
- Catalog search can suggest the column that should be used for color coding
    in the search results metadata