Add support for radio buttons based on a slot

[Fleker]

For a smart home sample project we want to have several exclusive options for a given smart home state backed by icons. Rather than writing our own radio-group element we'd like to use the existing radio-group and radio-button elements. This change adds an optional icon attribute and icon slot to the element which transforms the regular empty/filled buttons into a colorful icon.

See the updated demo for the design.

@proppy