Clean up the magnifying glass-related code.

[hwf1324]

Link to issue number:

Summary of the issue:

Clean up the magnifying glass-related code.

Description of user facing changes:

None

Description of developer facing changes:

Clean up the magnifying glass-related code.

Description of development approach:

• Remove duplicate screen curtain checks.
• Type hint: Added @override annotation.
• fix tyype hint
• Remove redundant return
• rename filter variable to currentFilter, Because it shadows a builtin functions.

Testing strategy:

Known issues with pull request:

None

Code Review Checklist:

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

[seanbudd]

Thanks @hwf1324