Improved dialog for deletion of multiple files

[hiddevanesch]

Fixes #9473

Created DeleteFileAction class that extends SimpleCommand. This class handles both the creation of the dialog that either shows a file name or the amount of files, depending on the selection in the LinkedFileViewModel, as well as the unlinking/deletion itself. Also changed the call in LinkedFilesEditor when Delete is pressed to use this new DeleteFileAction class.

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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.

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Deletion of a single file:

Deletion of multiple files:

[Siedlerchr]

Thanks for your contribution! Looks good to me. Please add a changelog entry!

[hiddevanesch]

@Siedlerchr

Thanks for your contribution! Looks good to me. Please add a changelog entry!

I've already added an entry to CHANGELOG.md, or do you mean something else?

[Siedlerchr]

Ah sorry, now I see the changelog entry. All fine. Seems like I overlooked that.

[calixtus]

Codewise looks good. One nitpick about the wording: Disk or drive? I'm totally unsure on this. Thinking of terms like "Google Drive" / "OneDrive" etc.

[hiddevanesch]

@calixtus

Codewise looks good. One nitpick about the wording: Disk or drive? I'm totally unsure on this. Thinking of terms like "Google Drive" / "OneDrive" etc.

I think "disk" was used already in the existing dialog. Therefore, we adapted our addition using this term.