Always select entry in main table when showing entry editor

[koppor]

In some code patghways, the entry editor was shown, but the main table selection was not updated.

IMHO if the entry editor is shown - there should be a single (!) entry in the main table focused: The entry currently edited.

(This PR contains some code cleanup from #12124 - the important change is the removal of explicit clearAndSelect calls and include them into showAndEdit)

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• 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.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.