Fixes for Catalogs plugin

	modified:   ginga/LayerImage.py
	modified:   ginga/gtkw/PluginManagerGtk.py
	modified:   ginga/gtkw/plugins/Catalogs.py
	modified:   ginga/gtkw/plugins/Header.py
	modified:   ginga/gtkw/plugins/Info.py
	modified:   ginga/misc/plugins/CatalogsBase.py
	modified:   ginga/qtw/ColorBar.py
	modified:   ginga/qtw/QtHelp.py
	modified:   ginga/qtw/plugins/Catalogs.py

- some bug fixes for Catalogs plugin for object lookup and plotting
  - now properly shows field selected for plotting color