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:
- [ ] Circle ROIs cause application to hang for me (I think this has been resolved in ginga)
- [ ] remove print statements
- [ ] Keep subset color synced if it changes after adding it to ginga widget
- [ ] fix "X" icon of ginga data layers, which implies they are disabled
- [ ] implement rgb mode
- [ ] add some ginga-specific tests

Would be nice:
- [ ] See if we can implement back/forward/home buttons
- [ ] 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
- [ ] 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