Align focused button outline to vscode

[msujew]

What it does

Closes #11191

Removes box shadows, etc. from theia-button elements and adds an outline instead

How to test

1. Open a dialog by - for example - trying to delete a file.
2. Assert that the buttons behave as in vscode in regards to hover/focus behavior.

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]

The changes look good to me. They are consistent with VSCode (browser), and produce no regressions in the highlighting of buttons in different contexts.