Refactored BibEntryTypesManager and moved ui to preferences

[calixtus]

This is part of the preferences to new pattern conversion project, but is not actually converting the preferences to the new preferences pattern.

Documentation changed in JabRef/user-documentation#450

Compulsory checks

Beta Give feedback

1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.