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 
Actually I take that back, because load_ui looks in a specific directory for a path. Still, we could add a path kwarg to load_ui to support PySide