UI profiler extension

[krassowski]

Draft of a profiler extension. Implements #119.

Milestones:

• Client-side benchmarking scaffold
• CSS rule/sheet benchmarking code
• Find potentially unused styles
• Scenarios:
  • main menu (open/switch),
  • tab switch,
  • completer,
  • focus switch
  • panel switch
  • scrolling
• allow to run all scenarios at once
• UI for triggering benchmarks:
  • expose selected options in UI when viewing past results
  • polish UI: the run panel did not get any options in the end, so it can be moved to bottom right.
  • enhancement: add number of elements matching each rule and performance gain/#elements ratio (not very useful)
• integration with CI
• PoC for self-profiling API

Follow up items (in future PRs):

• extensibility, either:
  • rewrite in terms of user-provided scenarios via simple composition of commands, or
  • expose token to allow registering a custom IScenario in extensions.
• add more scenarios: resizing panels, opening context menu
• add workspace presets ("Workspace with all types of widgets open", "Workspace with long notebooks", "Workspace with many long files")

[krassowski]

Taking it out of draft as all the major bits are in. I will still work on bits and pieces but I do not plan adding anything major in this PR.

[krassowski]

Binders:

• 3.5.0 https://mybinder.org/v2/gist/krassowski/bd2965131d665ea547e294d3c7ef66cd/HEAD?urlpath=lab
• 4.0.0a30 https://mybinder.org/v2/gist/krassowski/521bb655f99d9b001f8ffa262f6d7e02/HEAD?urlpath=lab

[krassowski]

Development will continue in https://github.com/jupyterlab/ui-profiler. I will open a new PR integrating jupyterlab-ui-profiler with benchmarks CI.