Allowing changing entry types of multiple entries

[ThisTestUser]

Fixes #8654
This allows for the right click option "Change entry type" to change the entry types of multiple entries at once.
Trying to change the entry type by first double clicking on the entry still behaves normally.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• 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.

[Siedlerchr]

Codewise lgtm already! Tested as well and works