Use more Lab core components

[bollwyvl]

Is your feature request related to a problem? Please describe.

The monolithic react component makes it very difficult to unwrap when something goes wrong and is not easily extensible by third- or first-party code without a prescriptive API. Additionally, hand-coding things to look like core components loses out on some of the benefits provided by lab, e.g. accessibility and themability improvements.

As a case in point, develop has currently picked up the seemingly-endemic issue of input tags thrashing on focus when value change.

Even if it was extensible, and robust, it would demand whatever downstream provider was using would need to be written in react.

Describe the solution you'd like

• Reorganize the top-level structure of the extension to use more core lab components
  • the top-level widget is a plain-old MainAreaWidget (with a regular toolbar, etc.)
  • this will improve "active" management, etc.
  • use the lab IFrame for the actual VideoChat
  • should further clean up the CSS and mouse behavior
• update the provider API (unreleased anyway) more like other lab patterns, with a createNew for room listing and creation
• move the various room UIs into their plugins
  • many of the leaf components will still be react-based tsx, but use lab's VDomRenderer

Describe alternatives you've considered

• somehow make react more composable
  • went down this route, and didn't find a lot of existing patterns that felt very good... this made the various Props objects even longer
• drop react altogether
  • again, already stuck with it in lab, and straight lumino or vanilla dom is annoying

Additional context

• branch: develop
• this is similar to the effort on jupyterlab-pullrequests

[bollwyvl]

This is mostly done now... to get all the way to e.g. a mimerenderer would take a bit more, but it's much better as of v0.6.0.