Update Open Editors widget UI

[msujew]

What it does

Minor changes to focus and hovering effects for the "Open Editors" widget and tree views in general, which were missing their outlines.

How to test

1. Open the "Open Editors" view container and add a few files to it
2. The left padding of all nodes should align to vscode
3. The selected node should always display its close icon
4. Hovering over the close icon should show the usual rounded border
5. Hovering over a dirty node should show the close icon.

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

[colin-grant-work]

@msujew, with your tabbar work merged, this should be good to go after a rebase?

[msujew]

@colin-grant-work Thanks for the reminder, I rebased my changes 👍

[colin-grant-work]

@msujew, I believe the scenario I described in my comment above is still causing trouble. I've removed my self-review request just for bookkeeping purposes - ping me when it's ready for another look.

[msujew]

@colin-grant-work I had time to get back to this. I removed the functional change and kept it limited to purely visual changes. In my newest change I also corrected the left padding of the tree nodes, which wasn't quite correct.

[colin-grant-work]

Changes and new interactions look good to me. 👍

[vince-fugnitto]

@msujew I noticed a couple of things when quickly trying out the functionality:

1. It seems as though the selection is not maintained similarly to the explorer, and vscode. In the case of the open-editors widget the selection is always passive:

open-editors-selection.mov

2. I noticed we did not have ellipsis for overflowing content in the open-editors, but it seems to be a regression as it used to work (tree-view: update styling when the tree row content cannot be fully displayed #7145). Not related to your changes but I thought I'd mention it here for visibility:

[msujew]

@vince-fugnitto Thanks for chiming in!

1. Yeah, that was originally the reason I created this PR, in my first draft (see this comment). However, that lead to some unexpected consequences, which is why I reverted the change. In the case of the file system explorer, the same thing actually happens. Double clicking a file removes focus from the file tree and puts it on the editor. The same thing is happening in the case of the open editors widget, just with a single click.
2. I see, that's good to know. We can fix this in a separate PR.

[vince-fugnitto]

The styling updates look much better 👍
I confirm that:

• the padding works well
• different file-icon themes display properly
• selection is maintained, and so is the close icon
• the close icon has consistent styling with other clickable icons present in the framework