Initial clean-up to produce glue-qt

[astrofrog]

This is part of https://github.com/glue-viz/glue/issues/2430

This transitions the code in this repository to just be the glue-qt package. Not at all complete for now, just experimental.

Known TODOs:

• [x] Add patches to make loading session files possible again
• [x] Update dependencies to remove many of the dependencies that are just needed for glue-core and to add glue-core itself
• [x] Get tests passing
• [x] Update documentation

[astrofrog]

I think this is ready so I'll go ahead and merge!