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/auto_update/src/update_notification.rs

@@ -4,7 +4,7 @@ use gpui::{
 };
 use menu::Cancel;
 use util::channel::ReleaseChannel;
-use workspace::ui::{h_stack, v_stack, Icon, IconElement, Label, StyledExt};
+use workspace::ui::{h_stack, v_stack, Icon, IconName, Label, StyledExt};
 
 pub struct UpdateNotification {
     version: SemanticVersion,
@@ -30,7 +30,7 @@ impl Render for UpdateNotification {
                     .child(
                         div()
                             .id("cancel")
-                            .child(IconElement::new(Icon::Close))
+                            .child(Icon::new(IconName::Close))
                             .cursor_pointer()
                             .on_click(cx.listener(|this, _, cx| this.dismiss(&menu::Cancel, cx))),
                     ),