Refactor viewer classes to split out Qt from non-Qt part, and define Jupyter viewers

[astrofrog]

VisPy now works inside Jupyter notebook/lab thanks to jupyter_rfb, so this PR aims to split out the Qt-specific parts of this package and define separate Qt and Jupyter viewers. Main tasks to do:

• [x] Develop ipyvuetify option panels similar to what we use for the default viewers in glue-jupyter and set up a notebook to try these out without specific viewers. For now have this live in glue-vispy-viewers
• [x] Split out Qt code from scatter viewer and add new Jupyter viewer
• [x] Split out Qt code from volume viewer and add new Jupyter viewer
• [x] Get selection tools to work properly
• [x] Adjust glue-jupyter to make it possible to choose vispy as a front-end for the 3D viewers
  • https://github.com/glue-viz/glue-jupyter/pull/453
• [x] Make it possible to run tests without Qt
• [ ] Add visual tests, perhaps in glue-jupyter since the infrastructure is there already
• [ ] Decide whether to move any code to glue-core/glue-jupyter/glue-qt