bdDetect: swap KEY_* constants with enum

[LeonarddeR]

Link to issue number:

None

Summary of the issue:

bdDetect is using old style string constants to distinguish device types.

Description of user facing changes

None.

Description of development approach

Replace with enum.

Testing strategy:

Test whether random braille display is still detected appropriately.

Known issues with pull request:

None known

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 @LeonarddeR