Image refactoring and ginga widget

cc @ejeschke

Here's a PR to coordinate and finalize ginga integration. This branch has been rebased against @ejeschke's branch and glue master, and also incorporates some additional housecleaning on the glue codebase. Remaining TODO items

Required:
- [x] Circle ROIs cause application to hang for me (I think this has been resolved in ginga)
- [x] remove print statements
- [x] Keep subset color synced if it changes after adding it to ginga widget
- [x] fix "X" icon of ginga data layers, which implies they are disabled
- [ ] implement rgb mode
- [ ] add some ginga-specific tests
- [x] sync visibility toggling
- [ ] sync zordering

Would be nice:
- [ ] See if we can implement back/forward/home buttons
- [x] refactor pv slice and profile tools so they can use either the Ginga or MPL image viewer
- [ ] Let users use existing colormap glue config item to customize ginga colormap selections
- [x] see if there's a faster way to enable sliding through cubes, by letting ginga slice through an ND dataset instead of pre-slicing it for ginga
- [ ] scatterplot overlays
- [x] make toolbar look more like Glue toolbar