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
Extract mpl-agnostic base class out of ImageClient
cleanup/document image client
Rename ax->axes