Refactor `OpenExternalFileAction`

[koppor]

Split org.jabref.gui.maintable.OpenExternalFileAction into OpenSingleExternalFileAction and OpenSelectedEntriesFilesAction

• The functionality of entry == null moves to OpenSelectedEntriesFilesAction
• No entry and linkedFile variables for OpenSelectedEntriesFilesAction
• Single constructors for both

• No CHANGELOG.md entry as this is not user facing.
• No tests needed

[athul2k2]

Hey, iam new to Open source contribution, look like i can help on this issue ..

[github-actions]

👋 Hey, looks like you’re eager to work on this issue—great! 🎉 It also looks like you skipped reading our CONTRIBUTING.md, which explains exactly how to participate. No worries, it happens to the best of us. Give it a read, and you’ll discover the ancient wisdom of assigning issues to yourself. Trust me, it’s worth it. 🚀

[Muskan244]

/assign-me