Align border-radius with vscode

[msujew]

What it does

VSCode has recently updated a lot of their UI to use border radius' (menus, menu bars, notifications, buttons, text boxes, quick input etc.). This change aligns Theia to this design decision.

How to test

Assert that the changed elements align with vscode in regards to their border design.

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]

Overall the changes look good to me, I noticed two missing areas when testing:

• the AlertMessage are not rounded (ex: when no workspace is present and we check the scm view)
• the theia-select (select input) are not rounded

[msujew]

Thanks Vince for looking it. I've adjusted those two (actually three see ebbbdc2) points.

[vince-fugnitto]

LGTM 👍
The rounded borders in the quick-open menus will likely be contributed by newer monaco versions.