Add progressbar and switch widgets to web platform.

[malikoth]

Add new widgets for the web platform:

• ProgressBar
• Switch

Also adds a pyodide import to toga_web.libs so we can use the create_proxy function to avoid having our proxies destroyed as seen here.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[malikoth]

I think briefcase broke... The output of the CI / tesbed (linux) (pull_request) action does not look like it would have been caused by this change:

Run cd testbed
  cd testbed
  xvfb-run -a -s "-screen 0 2048x1536x24" briefcase run linux --test 
  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
  env:
    min_python_version: 3.7
    max_python_version: 3.11

[testbed] Finalizing application configuration...
Targeting ubuntu:jammy (Vendor base debian)

Log saved to /home/runner/work/toga/toga/testbed/logs/briefcase.2023_04_25-22_02_08.run.log


Traceback (most recent call last):
  File "/home/runner/.local/bin/briefcase", line 8, in <module>
    sys.exit(main())
  File "/home/runner/.local/lib/python3.10/site-packages/briefcase/__main__.py", line 25, in main
    command(**options)
  File "/home/runner/.local/lib/python3.10/site-packages/briefcase/commands/run.py", line 273, in __call__
    self.finalize(app)
  File "/home/runner/.local/lib/python3.10/site-packages/briefcase/commands/base.py", line 529, in finalize
    self.finalize_app_config(app)
  File "/home/runner/.local/lib/python3.10/site-packages/briefcase/platforms/linux/system.py", line 161, in finalize_app_config
    if not self.use_docker:
AttributeError: 'LinuxSystemRunCommand' object has no attribute 'use_docker'
Error: Process completed with exit code 1.

[freakboy3742]

Whelp... that CI failure is a bit of a mess - it's going to be related to beeware/briefcase#1222.

[freakboy3742]

Awesome - thanks!