Fix Enum str mixup

[feerrenrut]

Link to issue number:

fixes #13109

Summary of the issue:

Thunderbird and Firefox were broken with PR #13096.

Description of how this pull request fixes the issue:

• The RelationType enum now also inherits from str, its values can now be used as a string value.
• Fixed conversion of plain str types to RelationType
• Introduced a faster approach to get relation targets of the relation type.

Testing strategy:

• Tested locally with Firefox.
• Ask reporters to verify fix.

Known issues with pull request:

None

Change log entries:

None

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

[feerrenrut]

One reporter has verified the PR resolves the issue.

[tapper82]

Tested in firefox 95 64 bit and Thunderbird Daily 96.0a1 (2021-11-30) (64-bit) Running on windows 11 64 bit.

[OzancanKaratas]

Looks good to me! Thank you for your work!

[LeonarddeR]

Works properly in Thunderbird for me as well.