Fix EL ResourceBundle preview panel

[NicolaIsotta]

Before:

After:

This also makes the "Open source in editor button" works and fixes the load of bundle files from the default package / root folder.

---

^Add meaningful description above

Click to collapse/expand PR instructions

By opening a pull request you confirm that, unless explicitly stated otherwise, the changes -

• are all your own work, and you have the right to contribute them.
• are contributed solely under the terms and conditions of the Apache License 2.0 (see section 5 of the license for more information).

Please make sure (eg. git log) that all commits have a valid name and email address for you in the Author field.

If you're a first time contributor, see the Contributing guidelines for more information.

If you're a committer, please label the PR before pressing "Create pull request" so that the right test jobs can run.

PR approval and merge checklist:

1. Was this PR correctly labeled, did the right tests run? When did they run?
2. Is this PR squashed?
3. Are author name / email address correct? Are co-authors correctly listed? Do the commit messages need updates?
4. Does the PR title and description still fit after the Nth iteration? Is the description sufficient to appear in the release notes?

If this PR targets the delivery branch: don't merge. (full wiki article)

[NicolaIsotta]

Marking this as draft as there's another fix I'd like to make about bundle resolution

[mbien]

nice! I like those quality of life improvements.