This PR renames the `IconElement` component to just `Icon`.
This better matches the rest of our components, as `IconElement` was the
only one using this naming convention.
The `Icon` enum has been renamed to `IconName` to free up the name.
I was trying to come up with a way that would allow rendering an
`Icon::Zed` directly (and thus make the `IconElement` a hidden part of
the API), but I couldn't come up with a way to do this cleanly.
Release Notes:
- N/A
