Tabs: Add support for horizontal scrolling

[ledermann]

I'm using horizontal scrollable tabs in my current project, so the active tab may be out of sight after connecting the controller.

This PR fixes this by changing two things:

• The active tab always gets the attribute aria-selected="true"
• Using scrollIntoView() ensures that the active tab is visible

References:

• https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-selected
• https://developer.mozilla.org/en-US/docs/Web/API/Element/ariaSelected
• https://developer.mozilla.org/de/docs/Web/API/Element/scrollIntoView