Re-write GUI in Qt and try and make it pluggable in Glue

I'd like to try and work on re-writing the GUI in Qt (the matplotlib one kind of sucks for a number of reasons) and at the same time I think it would be nice to explore how we can do this and make it usable in glue at the same time.

cc @keflavich @ChrisBeaumont 
@ChrisBeaumont - thanks! Indeed this PR was opened before that change. I will update this now.