Add tooltip for tab close button

[msujew]

What it does

Adds a tooltip/title for the tab close button. Previously it showed the tooltip of the tab, now either "Close" or "Unpin", depending on whether it is closable or pinned.

Also cleans up some className manipulation sorrounding the pin feature.

How to test

1. Hover over the close/unpin button
2. Assert that the tooltip is updated correctly based on the state
3. Assert that the pinning/unpinning feature hasn't experienced any regressions

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 confirm that the tooltip is correctly displayed, and there are no visible regressions.