Feature request: to_glue

@ChrisBeaumont this is for you, maybe @astrofrog too since you made the successful one-off pvextractor widget.

Would it be possible to open a cube in Glue, or at least with the Glue quick-viewer, from within an existing session?  I'm thinking of getting access to on-the-fly coordinate changes within glue, essentially, but starting from a regular old command line application.
I tried it a few times and it didn't block.  However, this function also didn't return, which is why I stopped returning `glue_app`.  I'll try again, though...