Improve document viewer

[LoayGhreeb]

Some improvements to the document viewer and search results tab

• Clicking on the page number to open the document viewer, it was displaying the first linked file of the selected entry instead of the correct file. Also without custom CSS.

• "Open file" was open all linked files to the entry.
  

• Added validation to the next and previous page buttons in the document viewer to disable them on the first and last pages.

• Close the document viewer when the selected entry does not have any linked files.

• Avoid opening multiple document viewers.

• Selecting multiple entries will add all linked files of the selected entries to the list of files.
  

Mandatory checks

• 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 so far. Cannot test right now but hope you did 😅