Space key not handled in close button

[mlewand]

1. Open Accessibility Checker manual test.
2. Open AC by pressing it's icon.
3. Press shift + tab twice to move the focus on close button.
4. Press space.

Expected

AC gets closed.

Actual

Key event does not get handled, and default browser behavior apply (scrolling the page).

[mlewand]

@f1ames could you please review this one?

[f1ames]

@Tade0 There are still unanswered comments from @mlewand, I also added one myself, please take a look there - #206.