Add synthDriverHandler.synthChanged extension point

[LeonarddeR]

Link to issue number:

Part of #14520

Description of development approach

added synthDriverHandler.synthChanged extension point, which behaves similar to braille.displayChanged. It includes an extension point.

Testing strategy:

Unit tests and manual testing using the python console.

Known issues with pull request:

None

Change log entries:

For Developers
- Added extension point: synthDriverHandler.synthChanged

Code Review Checklist:

• Pull Request description:
  • description is up to date
  • change log entries
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• API is compatible with existing add-ons.
• Documentation:
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• Security precautions taken.

[AppVeyorBot]

• PASS: Translation comments check.
• PASS: Unit tests.
• FAIL: Lint check. See test results for more information.
• FAIL: System tests (tags: installer NVDA). See test results for more information.
• Build (for testing PR): https://ci.appveyor.com/api/buildjobs/o11lrl0fmx4hqi89/artifacts/output/nvda_snapshot_pr14618-27642,d6c6b1f8.exe
• CI timing (mins):
  INIT 0.0,
  INSTALL_START 0.9,
  INSTALL_END 0.9,
  BUILD_START 0.0,
  BUILD_END 23.9,
  TESTSETUP_START 0.0,
  TESTSETUP_END 0.3,
  TEST_START 0.0,
  TEST_END 23.3,
  FINISH_END 0.2

See test results for failed build of commit d6c6b1f8e6