Changes to facilitate user remapping of bindings

	modified:   ginga/Bindings.py
	modified:   ginga/FitsImageCanvas.py
	modified:   ginga/gtkw/FitsImageGtk.py
	modified:   ginga/gtkw/GingaGtk.py
	modified:   ginga/misc/plugins/CatalogsBase.py
	modified:   ginga/qtw/FitsImageQt.py
	modified:   ginga/qtw/GingaQt.py

- bindings classes can now be set using class methods in FitsImageZoom
  - these are then inherited by any such widget
- fix to CatalogsBase to pan to picked stars in catalogs, if enabled