Added minimum window sizing for creating new entries

[ityyrm]

Closes #11944

Added minimum sizing for windows to create new entries.

Minimum window size for creating a new entry:

It was also observed that the same issue is present for creating a new entry from plain text, so minimal window sizing was also included for this.:

Mandatory checks

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

Okay, fine, please fix the checkstyle issue and it's okay from my side Import jabrefs's code style and select organize imports

[Siedlerchr]

do not push any new changes once it's added to the merge queue