[PowerLauncher] don't use Environment.Exit(0) and gracefully shutdown…

[yuyoyuppe]

… all threads

Summary of the Pull Request

• use CancellationTokenSource for event waiter threads
• don't use Environment.Exit(0)

Possible risks:

• slower shutdown

If it fixes the issue, we need to do the same for other modules, e.g. ColorPicker, FZE, OCR.

PR Checklist

• Closes: Watson crash
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

• opens w/ hotkey
• opens from oobe
• exit PowerToys runner and verify Run closes
• try taskkill /f /im PowerToys.exe (would crash on exit before) and verify Run closes