open-editors: better styling and visible decorations

[vladarama]

What it does

This PR fixes #10821 by changing the styling of the open-editors view to align more closely with the styling of the explorer itself. These changes include making the decorations always visible even when the view becomes too small and adding an ellipsis overflow to the filename and their paths.

Demo:

How to test

• Start Theia application -> Go to Explorer View -> Make the Open Editors visible
• Open or create new files, make some changes under source control
• Resize the Open Editors view and observe the better styling

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

LGTM 👍

I confirmed that the resizing behavior is correct, and there are no visible regressions from what I can tell.