Support for variants in buttons

It is now possible to set a variant on a button, leading to a semantic change of appearance.

Example:

<x-btn variant="default">default</x-btn>
<x-btn variant="primary">primary</x-btn>
<x-btn variant="success">success</x-btn>
<x-btn variant="neutral">neutral</x-btn>
<x-btn variant="warning">warning</x-btn>
<x-btn variant="danger">danger</x-btn>
<x-btn variant="text">text</x-btn>