First attempt at integrating a Ginga viewer into Glue
Updates to ginga viewer option

- now uses the icons for the toolbar and exclusive mode for tool choice
- improvement in rendering speed of redraws with data subsets
Small updates
Changed to using image overlays feature
Further partial updates
Extract mpl-agnostic base class out of ImageClient
First-pass refactor of Ginga into glue-style artist layers
CLeanup
After client/artist reorg fixes for ginga viewer

- fixed delete layer (except base data layer)
- fixed visibility layer (except base data layer)
Improved toolbar, now closer to regular Image Viewer
Fix for recent changes to ginga

- now can toggle visibility of image layer
- removed or commented out debug print statements
Merge branch 'ginga' of https://github.com/ChrisBeaumont/glue into ginga-widget

Conflicts:
	glue/clients/ginga_client.py
	glue/clients/image_client.py
	glue/qt/widgets/ginga_widget.py
	glue/qt/widgets/image_widget.py