Rename IconElement to just Icon (#3974)

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

crates/ui/src/prelude.rs

@@ -15,6 +15,6 @@ pub use crate::{h_stack, v_stack};
 pub use crate::{Button, ButtonSize, ButtonStyle, IconButton, SelectableButton};
 pub use crate::{ButtonCommon, Color, StyledExt};
 pub use crate::{Headline, HeadlineSize};
-pub use crate::{Icon, IconElement, IconPosition, IconSize};
+pub use crate::{Icon, IconName, IconPosition, IconSize};
 pub use crate::{Label, LabelCommon, LabelSize, LineHeightStyle};
 pub use theme::ActiveTheme;