Reflect icon property to attribute

[mgiuffrida]

It's nice to style iron-icons based on which icon they show:

iron-icon[icon='done'] {
  --iron-icon-fill-color: var(--google-green-500);
}

However, if the icon property is changed dynamically, the new style rules won't apply (even after updateStyles) because the attribute hasn't changed.

[mgiuffrida]

/cc @azakus @kevinpschaaf

Would this have performance implications? iron-icon tends to be a high-frequency element, but the icon rarely changes, so it would mostly be an issue at startup.

[mgiuffrida]

rebased