Fix 6966 multiple file open

[hugomfandrade]

• 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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

Created a simple command for opening multiple files when one or multiple entries are selected, and added it to the right menu. One single entry can have multiple files and this command (OpenMultipleExternalFilesAction) iterates not only over the selected entries, but also the files of each entry, and opens them all.
A security check was added so that the maximum number of files that can be open are 20.
The following images illustrate the solution

https://github.com/CES21-Group-G/resources/blob/master/fix02/fix02-03.png
https://github.com/CES21-Group-G/resources/blob/master/fix02/fix02-04.png

[Siedlerchr]

Please check the next time if there is already a PR open, in this case it was #7709