Add support for 3D VisPy viewers

[astrofrog]

This exposes the vispy viewers as optional widgets for scatter3d and volshow, as alternatives to ipyvolume.

This requires:

• Refactor viewer classes to split out Qt from non-Qt part, and define Jupyter viewers glue-vispy-viewers#381

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 86.04%. Comparing base (015b428) to head (488287d).
Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
glue_jupyter/app.py	55.55%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
- Coverage   86.15%   86.04%   -0.12%     
==========================================
  Files          90       90              
  Lines        5209     5223      +14     
==========================================
+ Hits         4488     4494       +6     
- Misses        721      729       +8     

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